Absorbent components having a fluid acquisition zone

ABSTRACT

The disclosure relates to an absorbent core capable of absorbing discharged aqueous body fluids. The core contains a swellable fluid storage material that forms, upon contact with aqueous body fluids, a fluid acquisition zone. The fluid acquisition zone is located remote from the user, to avoid flowback or rewetting of the topsheet of an article containing the core. The disclosure further relates to an absorbent article useful for absorbing discharged aqueous body fluids that comprises: A) a fluid pervious topsheet; B) a backsheet; and C) an absorbent core, as described above, positioned between the topsheet and the backsheet.

TECHNICAL FIELD

This application relates to absorbent cores useful in absorbent articles such as diapers, adult incontinence pads, sanitary napkins, and the like capable of handling multiple discharges of aqueous body fluids. The application particularly relates to articles comprising an absorbent core having a fluid acquistion zone, located remote from the wearer, formed by the swelling of a fluid storage material.

BACKGROUND OF THE INVENTION

The development of highly absorbent members for use as disposable diapers, adult incontinence pads and briefs, and catamenial products such as sanitary napkins, are the subject of substantial commercial interest. A highly desired characteristic for such products is thinness. For example, thinner diapers are less bulky to wear, fit better under clothing, and are less noticeable. They are also more compact in the package, making the diapers easier for the consumer to carry and store. Compactness in packaging also results in reduced distribution costs for the manufacturer and distributor, including less shelf space required in the store per diaper unit.

The ability to provide thinner absorbent articles such as diapers has been contingent on the ability to develop relatively thin absorbent cores or structures that can acquire and store large quantities of discharged body fluids, in particular urine. In this regard, the use of certain absorbent polymers often referred to as "hydrogels," "superabsorbents" or "hydrocolloid" material has been particularly important. See, for example, U.S. Pat. No. 3,669,103 (Harper et al), issued Jun. 13, 1972, and U.S. Pat. No. 3,670,731 (Harmon), issued Jun. 20, 1972, that disclose the use of such absorbent polymers (hereafter "hydrogel-forming absorbent polymers") in absorbent articles. Indeed, the development of thinner diapers has been the direct consequence of thinner absorbent cores that take advantage of the ability of these hydrogel-forming absorbent polymers to absorb large quantities of discharged aqueous body fluids, typically when used in combination with a fibrous matrix. See, for example, U.S. Pat. No. 4,673,402 (Weisman et al), issued Jun. 16, 1987 and U.S. Pat. No. 4,935,022 (Lash et al), issued Jun. 19, 1990, that disclose dual-layer core structures comprising a fibrous matrix and hydrogel-forming absorbent polymers useful in fashioning thin, compact, nonbulky diapers.

Prior to the use of these hydrogel-forming absorbent polymers, it was general practice to form absorbent structures, such as those suitable for use in infant diapers, entirely from wood pulp fluff. Given the relatively low amount of fluid absorbed by wood pulp fluff on a gram of fluid absorbed per gram of wood pulp fluff, it was necessary to employ relatively large quantities of wood pulp fluff, thus necessitating the use of relatively bulky, thick absorbent structures. The introduction of these hydrogel-forming absorbent polymers into such structures has allowed the use of less wood pulp fluff. These hydrogel-forming absorbent polymers are superior to fluff in their ability to absorb large volumes of aqueous body fluids, such as urine (i.e., at least about 15 g/g), thus making smaller, thinner absorbent structures feasible.

Prior absorbent structures have generally comprised relatively low amounts (e.g., less than about 50% by weight) of these hydrogel-forming absorbent polymers. See, for example, U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989 (preferably from about 9 to about 50% hydrogel-forming absorbent polymer in the fibrous matrix). There are several reasons for this. The hydrogel-forming absorbent polymers employed in prior absorbent structures have generally not had an absorption rate that would allow them to quickly absorb body fluids, especially in "gush" situations. This has necessitated the inclusion of fibers, typically wood pulp fibers, to serve as temporary reservoirs to hold the discharged fluids until absorbed by the hydrogel-forming absorbent polymer.

More importantly, many of the known hydrogel-forming absorbent polymers exhibited gel blocking. "Gel blocking" occurs when particles of the hydrogel-forming absorbent polymer are wetted and the particles swell so as to inhibit fluid transmission to other regions of the absorbent structure. Wetting of these other regions of the absorbent member therefore takes place via a very slow diffusion process. In practical terms, this means acquisition of fluids by the absorbent structure is much slower than the rate at which fluids are discharged, especially in gush situations. Leakage from the absorbent article can take place well before the particles of hydrogel-forming absorbent polymer in the absorbent member are fully saturated or before the fluid can diffuse or wick past the "blocking" particles into the rest of the absorbent member. Gel blocking can be a particularly acute problem if the particles of hydrogel-forming absorbent polymer do not have adequate gel strength and deform or spread under stress once the particles swell with absorbed fluid. See U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989.

This gel blocking phenomena has typically necessitated the use of a fibrous matrix in which are dispersed the particles of hydrogel-forming absorbent polymer. This fibrous matrix keeps the particles of hydrogel-forming absorbent polymer separated from one another. This fibrous matrix also provides a capillary structure that allows fluid to reach the hydrogel-forming absorbent polymer located in regions remote from the initial fluid discharge point. See U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989. However, dispersing the hydrogel-forming absorbent polymer in a fibrous matrix at relatively low concentrations in order to minimize or avoid gel blocking can lower the overall fluid storage capacity of thinner absorbent structures. Also, absorbent cores comprising hydrogel-forming absorbent polymers dispersed uniformly throughout the fibrous matrix will typically not have the ability to rapidly acquire and distribute fluids during "gush" situations or when the core has become saturated from prior discharges of body fluids.

The need for rapidly acquiring and distributing discharge body fluids has led to the development of dual-layer core structures noted above. These dual-layer core structures basically comprise: (1) an upper fibrous layer adjacent to the fluid pervious topsheet that is substantially free of hydrogel-forming absorbent polymers that acquires the discharged fluid; and (2) a lower layer that stores this acquired fluid and is typically either: (a) a fibrous matrix having hydrogel-forming absorbent polymers uniformly dispersed therein; or (b) a laminate structure where hydrogel-forming absorbent polymer is between two tissue layers. See, for example, U.S. Pat. No. 4,673,402 (Weisman et al), issued Jun. 16, 1987. See also U.S. Pat. No. 4,935,022 (Lash et al), issued Jun. 19, 1990 and U.S. Pat. No. 5,217,445 (Young et al), issued Jun. 8, 1993, where certain chemically stiffened curly, twisted cellulosic fibers are used in this upper layer to provide improved acquisition and distribution performance. Another variation is to "profile" the absorbent core such that there is an acquisition zone substantially free of hydrogel-forming absorbent polymers in the fluid discharge area and a storage area having dispersed therein hydrogel-forming absorbent polymers that is in fluid communication with the acquisition zone. See U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989 and U.S. Pat. No. 5,047,023 (Berg), issued Sep. 10, 1991.

Even with the fluid handling improvements provided by these prior absorbent designs, it has been found that the ability to readily acquire fluid diminishes rapidly as the absorbent core becomes saturated with aqueous body fluids. This occurs because the void spaces between fibers and the hydrogel-forming absorbent polymers in the absorbent core become partially filled with fluid during the first "gush" and therefore can not rapidly accept the necessary volume of fluid during subsequent "gushes". Furthermore, the risk of leakage increases as the number of loads increases.

Another problem that can occur with some prior absorbent core designs is a phenomenon referred to as "rewet." Rewet occurs when there is acquired fluid that is freely mobile and available in that portion of the absorbent core adjacent the topsheet. This is typically experienced as the absorbent core becomes saturated with acquired fluid. Under mechanical pressure from the wearer of the article, this mobile fluid is pumped out of the absorbent core and upwards through the topsheet. As a result, the topsheet becomes "rewetted" with this pumped fluid such that there is not adequate topsheet dryness.

Accordingly, it would also be desirable to provide an absorbent core that: (1) has an absorbent material, capable of swelling upon absorbing discharged body fluid to form a fluid acquisition zone, in the absorbent core for desired total fluid capacity and thinness; (2) is able to acquire discharged fluid rapidly during "gush" situations, even when the core has become saturated in the loading area from prior discharges of fluids; and (3) when incorporated into an absorbent article, preferably minimizes rewetting of the topsheet.

SUMMARY OF THE INVENTION

In one aspect, the present invention relates to an absorbent core capable of absorbing discharged body fluids, the absorbent core comprising:

(1) a fluid acquisition/distribution component capable of receiving aqueous fluids in the fluid discharge region of the absorbent core, wherein the fluid acquisition/distribution component either

(a) contains less than about 50%, by weight of the acquisition/distribution component, of absorbent hydrogel-forming polymer; or

(b) contains at least 50%, by weight of the acquisition/distribution component, of absorbent hydrogel-forming polymer, the absorbent hydrogel-forming polymer having a Saline Flow Conductivity (SFC) value of at least about 50×10⁻⁷ cm³ sec/g; and

(2) at least one fluid storage component positioned at least partially underneath and in fluid communication with the fluid acquisition/distribution component, said at least one fluid storage component being capable of expanding in the z-direction when contacted with aqueous body fluids so as to form a fluid acquisition zone.

In another aspect, the present invention relates to an absorbent core comprising:

(1) an upper fluid acquisition/distribution component capable of receiving aqueous fluids in the fluid discharge region of the absorbent core, the upper fluid acquisition component having from 0 to about 15%, by weight, of absorbent hydrogel-forming polymer;

(2) a permeable upper fluid storage component at least partially underneath and in fluid communication with said upper fluid acquisition component, said upper fluid storage component being permeable to the flow of aqueous body fluids in the z-direction;

(3) a lower fluid acquisition/distribution component capable of acquiring and transporting aqueous body fluids, the lower fluid acquisition/distribution component being positioned at least partially underneath and in fluid communication with said upper fluid storage component; and

(4) at least one lower fluid storage component positioned at least partially underneath and in fluid communication with the lower fluid acquisition/distribution component, said at least one lower fluid storage component being capable of expanding in the z-direction when contacted with aqueous body fluids so as to form a fluid acquisition zone.

The present invention further relates to an absorbent article useful for absorbing discharged aqueous body fluids that comprises: A) a fluid pervious topsheet; B) a backsheet; and C) an absorbent core of the present invention positioned between the topsheet and the backsheet.

The absorbent articles of the present invention have an improved ability to rapidly acquire, distribute and store discharged body fluids due to the presence of: (1) the fluid acquisition zone that is formed by the swellable fluid storage component; and (2) the permeable fluid acquisition/distribution component that is capable of receiving "gushes" of body fluids and rapidly desorbing the fluid into the lower fluid storage components and the fluid acquisition zone, thereby being capable of receiving the next fluid gush. This is especially important when portions of the absorbent core become saturated from prior multiple discharges of such fluids. The absorbent articles of the present invention also minimize rewetting of the topsheet, as a result of locating the fluid acquistion zone remote from the user. That is, by locating the fluid acquisition zone/swellable storage component lower in the article, body fluids are immobilized away from the topsheet, to prevent flowback of the fluids to contact the wearer's skin. This provides good skin dryness for the wearer of an article containing the cores of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of an absorbent article according to the present invention where the topsheet is transparent so as to more clearly show the absorbent core.

FIG. 2 is a cross-sectional view taken along line 2--2 of FIG. 1.

FIG. 3 is a cross-sectional view of an absorbent article showing an alternative absorbent core according to the present invention.

FIG. 4 is a cross-sectional view of an absorbent article showing another alternative absorbent core according to the present invention.

FIG. 5 is a cross-sectional view of an absorbent article showing an alternative absorbent core according to the present invention.

FIG. 6 is a cross-sectional view of an absorbent article showing an alternative absorbent core according to the present invention.

FIG. 7 represents a schematic view of an apparatus for measuring the Saline Flow Conductivity (SFC) value of hydrogel-forming absorbent polymers which are optionally useful in the present invention.

FIG. 8 represents an enlarged sectional view of the piston/cylinder assembly shown in FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

A. Definitions

As used herein, the term "aqueous body fluids" includes urine, menses and vaginal discharges.

As used herein, the term "Z-dimension" refers to the dimension orthogonal to the length and width of the member, core or article. The Z-dimension usually corresponds to the thickness of the member, core or article.

As used herein, the term "X-Y dimension" refers to the plane orthogonal to the thickness of the member, core or article. The X and Y dimensions correspond to the length and width, respectively, of the member, core or article.

As used herein, the term "absorbent core" refers to the component of the absorbent article that is primarily responsible for fluid handling properties of the article, including acquiring, transporting, distributing and storing aqueous body fluids. As such, the absorbent core typically does not include the topsheet or backsheet of the absorbent article.

As used herein, "direct communiciation" means that fluid can transfer readily between two absorbent article components (e.g., the fluid aquisition/distribution component and the swellable fluid storage component) without substantial accumulation, transport, or restriction by an interposed layer. For example, tissues, nonwoven webs, construction adhesives, and the like may be present between two core layers or components while maintaining "direct communication", as long as they do not substantially accumulate (store), transport (wick), or restrict the fluid as it passes between such layers or components.

As used herein, the term "load" or "gush" generally refers to an insult or depostion of urine or other bodily fluid that would typically result during use. The term load may also refer to the total amount of liquid contained in an absorbent article, but typically refers to one fluid insult.

As use herein, the term "layer" refers to an absorbent member whose primary dimension is X-Y, i.e., along its length and width. It should be understood that the term layer is not necessarily limited to single layers or sheets of material. Thus the layer can comprise laminates or combinations of several sheets or webs of the requisite type of materials. Accordingly, the term "layer" includes the terms "layers" and "layered."

For purposes of this invention, it should also be understood that the term "upper" refers to absorbent core components, such as layers, that are nearest to the wearer of the absorbent article, and are typically nearer the topsheet of an absorbent article; conversely, the term "lower" refers to absorbent core components that are furthermost away from the wearer of the absorbent article and are typically nearer the backsheet.

As used herein, the term "comprising" means various components, steps and the like can be conjointly employed according to the present invention. Accordingly, the term "comprising" encompasses the more restrictive terms "consisting essentially of" and "consisting of," these latter, more restrictive terms having their standard meaning as understood in the art.

All percentages, ratios and proportions used herein are by weight unless otherwise specified.

B. Absorbent Core Components

Exemplary core components useful in achieving improved acquisition performance are described below.

1. Fluid Acquisition/Distribution Component(s)

The cores of the present invention comprise at least one fluid acquisition/distribution component. In one aspect, the cores comprise an upper and a lower acquisition/distribution component. In either case, however, the cores comprise such a component that will be in direct fluid communication with the absorbent article's topsheet.

The fluid acquisition/distribution component(s) can provide a variety of functions in the absorbent cores of the present invention. One function, particularly of the acquisition component adjacent the topsheet, is to initially acquire the discharged body fluids. Another key function is to transport and distribute these acquired fluids to other absorbent core components, and in particular the fluid storage components of the absorbent core. In some instances, the fluid acquisition/distribution components according to the present invention can include at least some hydrogel-forming absorbent polymer and thus provide some fluid storage capacity for the absorbent core. However, for the acquisition/distribution component in direct fluid communication with the topsheet, it is essential that this component be sufficiently permeable to allow rapid penetration of fluid to components lower in the article. As such, it is essential that the acquisition/distribution component located in direct fluid communication with the topsheet either (a) contain less than about 50%, by weight of the acquisition/distribution component, of absorbent hydrogel-forming polymer; or (b) contain at least 50%, by weight of the acquisition/distribution component, of absorbent hydrogel-forming polymer, provided that the polymer must have a Saline Flow Conductivity (SFC) value of at least about 50×10⁻⁷ cm³ sec/g, as measured according to the Test Method section discussed below. Where the component contains at least 50% hydrogel-forming polymer, it is preferred that the SFC value is at least about 100×10⁻⁷ cm³ sec/g. More preferred, however, is where this component is less than about 30%, by weight, of hydrogel-forming polymer.

The detailed description that follows refers generally to the materials useful as the acquistion/distribution component(s). Though referred to in the singular, the description applies to both components where an upper and lower acquisition/distribution are utilized. Where an upper and lower component are used, they may be the substantially the same, or they may be different, so long as the requirements of permeability for the upper component are satisfied.

The fluid acquisition/distribution component of the present invention can comprise a variety of fibrous materials that form fibrous webs or fibrous matrices. Fibers useful in the fluid acquisition/distribution component include those that are naturally occurring fibers (modified or unmodified), as well as synthetically made fibers. Examples of suitable unmodified/modified naturally occurring fibers include cotton, Esparto grass, bagasse, kemp, flax, silk, wool, wood pulp, chemically modified wood pulp, jute, rayon, ethyl cellulose, and cellulose acetate. Suitable synthetic fibers can be made from polyvinyl chloride, polyvinyl fluoride, polytetrafluoroethylene, polyvinylidene chloride, polyacrylics such as ORLON®), polyvinyl acetate, polyethylvinyl acetate, non-soluble or soluble polyvinyl alcohol, polyolefins such as polyethylene (e.g., PULPEX®) and polypropylene, polyamides such as nylon, polyesters such as DACRON® or KODEL®, polyurethanes, polystyrenes, and the like. The fibers used can comprise solely naturally occurring fibers, solely synthetic fibers, or any compatible combination of naturally occurring and synthetic fibers.

The fibers useful in fluid acquisition/distribution components of the present invention can be hydrophilic, hydrophobic fibers that are made hydrophilic, or can be a combination of both hydrophilic and hydrophilized hydrophobic fibers. As used herein, the term "hydrophilic" describes fibers, or surfaces of fibers, that are wettable by aqueous fluids (e.g., aqueous body fluids) deposited on these fibers. Hydrophilicity and wettability are typically defined in terms of contact angle and the surface tension of the fluids and solids involved. This is discussed in detail in the American Chemical Society publication entitled Contact Angle, Wettability and Adhesion, edited by Robert F. Gould (Copyright 1964). A fiber, or surface of a fiber, is said to be wetted by a fluid (i.e., hydrophilic) when either the contact angle between the fluid and the fiber, or its surface, is less than 90°, or when the fluid tends to spread spontaneously across the surface of the fiber, both conditions normally co-existing. Conversely, a fiber or surface is considered to be hydrophobic if the contact angle is greater than 90° and the fluid does not spread spontaneously across the surface of the fiber.

Other fibers useful as the fluid aquistition/distribution component are hydrophobic fibers that are wettable due to their geometry. Such fibers include "capillary channel fibers" such as those described in U.S. Pat. No. 5,200,248, to Thompson et al. and U.S. Pat. No. 5,268,229, to Phillips et al, both of which are incorporated by reference herein.

Suitable hydrophilic fibers for use in the present invention include cellulosic fibers, modified cellulosic fibers, rayon, polyester fibers such as polyethylene terephthalate (e.g., DACRON®), hydrophilic nylon (HYDROFIL®), and the like. Suitable hydrophilic fibers can also be obtained by hydrophilizing hydrophobic fibers, such as surfactant-treated or silica-treated thermoplastic fibers derived from, for example, polyolefins such as polyethylene or polypropylene, polyacrylics, polyamides, polystyrenes, polyurethanes and the like. For reasons of availability and cost, cellulosic fibers, in particular wood pulp fibers, are preferred for use in the present invention.

Suitable wood pulp fibers can be obtained from well-known chemical processes such as the Kraft and sulfite processes. It is especially preferred to derive these wood pulp fibers from southern soft woods due to their premium absorbency characteristics. These wood pulp fibers can also be obtained from mechanical processes, such as ground wood, refiner mechanical, thermomechanical, chemimechanical, and chemi-thermomechanical pulp processes. Recycled or secondary wood pulp fibers, as well as bleached and unbleached wood pulp fibers, can be used.

A desirable source of hydrophilic fibers for use in the present invention are chemically stiffened cellulosic fibers. As used herein, the term "chemically stiffened cellulosic fibers" means cellulosic fibers that have been stiffened by chemical means to increase the stiffness of the fibers under both dry and aqueous conditions. Such means can include the addition of a chemical stiffening agent that, for example, coats and/or impregnates the fibers. Such means can also include the stiffening of the fibers by altering the chemical structure, e.g., by crosslinking polymer chains.

Polymeric stiffening agents that can coat or impregnate the cellulosic fibers include: cationic modified starches having nitrogen-containing groups (e.g., amino groups) such as those available from National Starch and Chemical Corp., Bridgewater, N.J., USA; latexes; wet strength resins such as polyamide-epichlorohydrin resin (e.g., Kymene®557H, Hercules, Inc. Wilmington, Del., U.S.A.), polyacrylamide resins described, for example, in U.S. Pat. No. 3,556,932 (Coscia et al), issued Jan. 19, 1971; commercially available polyacrylamides marketed by American Cyanamid Co., Stamford, Conn., U.S.A., under the tradename Parez® 631 NC; urea formaldehyde and melamine formaldehyde resins, and polyethylenimine resins. A general dissertation on wet strength resins utilized in the paper art, and generally applicable herein, can be found in TAPPI monograph series No. 29. "Wet Strength in Paper and Paperboard", Technical Association of the Pulp and Paper Industry (New York, 1965).

These fibers can also be stiffened by chemical reaction. For example, crosslinking agents can be applied to the fibers that, subsequent to application, are caused to chemically form intrafiber crosslink bonds. These crosslink bonds can increase the stiffness of the fibers. While the utilization of intrafiber crosslink bonds to chemically stiffen the fiber is preferred, it is not meant to exclude other types of reactions for chemical stiffening of the fibers. Fibers stiffened by crosslink bonds in individualized form (i.e., the individualized stiffened fibers, as well as processes for their preparation) are disclosed, for example, in U.S. Pat. No. 3,224,926 (Bernardin), issued Dec. 21, 1965; U.S. Pat. No. 3,440,135 (Chung), issued Apr. 22, 1969; U.S. Pat. No. 3,932,209 (Chatterjee), issued Jan. 13, 1976; and U.S. Pat. No. 4,035,147 (Sangenis et al), issued Jul. 12, 1977. More preferred stiffened fibers are disclosed in U.S. Pat. No. 4,822,453 (Dean et al), issued Apr. 18, 1989; U.S. Pat. No. 4,888,093 (Dean et al), issued Dec. 19, 1989; U.S. Pat. No. 4,898,642 (Moore et al), issued Feb. 6, 1990; and U.S. Pat. No. 5,137,537 (Herron et al), issued Aug. 11, 1992, all of which are incorporated by reference. In the more preferred stiffened fibers, chemical processing includes intrafiber crosslinking with crosslinking agents while such fibers are in a relatively dehydrated, defibrated (i.e., individualized), twisted, curled condition. See, for example, U.S. Pat. No. 4,898,642.

These chemically stiffened cellulosic fibers have certain properties that can make them particularly useful in fluid acquisition/distribution components according to the present invention, relative to unstiffened cellulosic fibers. In addition to being hydrophilic, these stiffened fibers have unique combinations of stiffness and resiliency. This allows thermally bonded fluid acquisition/distribution components made with these fibers to maintain high levels of absorptivity, and to exhibit high levels of resiliency and an expansionary responsiveness to wetting. In particular, the resiliency of these stiffened fibers enables the fluid acquisition/distribution component to better maintain its capillary structure in the presence of both fluid and compressive forces normally encountered during use and are thus more resistant to collapse.

In the case of thermally bonded fluid acquisition/distribution components useful in the present invention, a thermoplastic material is included with the fibers. Upon melting, at least a portion of this thermoplastic material migrates to the intersections of the fibers, typically due to interfiber capillary gradients. These intersections become bond sites for the thermoplastic material. When cooled, the thermoplastic materials at these intersections solidify to form the bond sites that hold the matrix or web of fibers together in each of the respective layers.

Amongst its various effects, bonding at these fiber intersections increases the overall compressive modulus and strength of the resulting thermally bonded fluid acquisition/distribution component. In the case of the chemically stiffened cellulosic fibers, the melting and migration of the thermoplastic material also has the effect of increasing the average pore size of the resultant web, while maintaining the density and basis weight of the web as originally formed. This can improve the fluid acquisition properties of the thermally bonded fluid distribution component upon initial discharges, due to improved fluid permeability, and upon subsequent discharges, due to the combined ability of the stiffened fibers to retain their stiffness upon wetting and the ability of the thermoplastic material to remain bonded at the fiber intersections upon wetting and upon wet compression. In net, thermally bonded webs of stiffened fibers retain their original overall volume, but with the volumetric regions previously occupied by the thermoplastic material becoming open to thus increase the average interfiber capillary pore size.

Thermoplastic materials useful in fluid distribution components of the present invention can be in any of a variety of forms including particulates, fibers, or combinations of particulates and fibers. Thermoplastic fibers are a particularly preferred form because of their ability to form numerous interfiber bond sites. Suitable thermoplastic materials can be made from any thermoplastic polymer that can be melted at temperatures that will not extensively damage the fibers that comprise the primary web or matrix of each layer. Preferably, the melting point of this thermoplastic material will be less than about 190° C., and preferably between about 75° C. and about 175° C. In any event, the melting point of this thermoplastic material should be no lower than the temperature at which the thermally bonded absorbent structures, when used in absorbent articles, are likely to be stored. The melting point of the thermoplastic material is typically no lower than about 5° C.

The thermoplastic materials, and in particular the thermoplastic fibers, can be made from a variety of thermoplastic polymers, including polyolefins such as polyethylene (e.g., PULPEX®) and polypropylene, polyesters, copolyesters, polyvinyl acetate, polyethylvinyl acetate, polyvinyl chloride, polyvinylidene chloride, polyacrylics, polyamides, copolyamides, polystyrenes, polyurethanes and copolymers of any of the foregoing such as vinyl chloride/vinyl acetate, and the like. One preferred thermoplastic binder fiber is PLEXAFIL® polyethylene microfibers (made by DuPont) that are also available as an about 20% blend with 80% cellulosic fibers sold under the tradename KITTYHAWK® (made by Weyerhaeuser Co.). Depending upon the desired characteristics for the resulting thermally bonded absorbent member, suitable thermoplastic materials include hydrophobic fibers that have been made hydrophilic, such as surfactant-treated or silica-treated thermoplastic fibers derived from, for example, polyolefins such as polyethylene or polypropylene, polyacrylics, polyamides, polystyrenes, polyurethanes and the like. The surface of the hydrophobic thermoplastic fiber can be rendered hydrophilic by treatment with a surfactant, such as a nonionic or anionic surfactant, e.g., by spraying the fiber with a surfactant, by dipping the fiber into a surfactant or by including the surfactant as part of the polymer melt in producing the thermoplastic fiber. Upon melting and resolidification, the surfactant will tend to remain at the surfaces of the thermoplastic fiber. Suitable surfactants include nonionic surfactants such as Brij® 76 manufactured by ICI Americas, Inc. of Wilmington, Del., and various surfactants sold under the Pegosperse® trademark by Glyco Chemical, Inc. of Greenwich, Conn. Besides nonionic surfactants, anionic surfactants can also be used. These surfactants can be applied to the thermoplastic fibers at levels of, for example, from about 0.2 to about 1 g. per sq. of centimeter of thermoplastic fiber.

Suitable thermoplastic fibers can be made from a single polymer (monocomponent fibers), or can be made from more than one polymer (e.g., bicomponent fibers). As used herein, "bicomponent fibers" refers to thermoplastic fibers that comprise a core fiber made from one polymer that is encased within a thermoplastic sheath made from a different polymer. The polymer comprising the sheath often melts at a different, typically lower, temperature than the polymer comprising the core. As a result, these bicomponent fibers provide thermal bonding due to melting of the sheath polymer, while retaining the desirable strength characteristics of the core polymer.

Suitable bicomponent fibers can include sheath/core fibers having the following polymer combinations: polyethylene/polypropylene, polyethylvinyl acetate/polypropylene, polyethylene/polyester, polypropylene/polyester, copolyester/polyester, and the like. Particularly suitable bicomponent thermoplastic fibers for use herein are those having a polypropylene or polyester core, and a lower melting copolyester, polyethylvinyl acetate or polyethylene sheath (e.g., DANAKLON®, CELBOND® or CHISSO® bicomponent fibers). These bicomponent fibers can be concentric or eccentric. As used herein, the terms "concentric" and "eccentric" refer to whether the sheath has a thickness that is even, or uneven, through the cross-sectional area of the bicomponent fiber. Eccentric bicomponent fibers can be desirable in providing more compressive strength at lower fiber thicknesses. Suitable bicomponent fibers for use herein can be either uncrimped (i.e. unbent) or crimped (i.e. bent). Bicomponent fibers can be crimped by typical textile means such as, for example, a stuffer box method or the gear crimp method to achieve a predominantly two-dimensional or "flat" crimp.

In the case of thermoplastic fibers, their length can vary depending upon the articular melt point and other properties desired for these fibers. Typically, these thermoplastic fibers have a length from about 0.3 to about 7.5 cm long, preferably from about 0.4 to about 3.0 cm long, and most preferably from about 0.6 to about 1.2 cm long. The properties, including melt point, of these thermoplastic fibers can also be adjusted by varying the diameter (caliper) of the fibers. The diameter of these thermoplastic fibers is typically defined in terms of either denier (grams per 9000 meters) or decitex (grams per 10,000 meters). Suitable bicomponent thermoplastic fibers can have a decitex in the range from about 1.0 to about 20, preferably from about 1.4 to about 10, and most preferably from about 1.7 to about 3.3.

The compressive modulus of these thermoplastic materials, and especially that of the thermoplastic fibers, can also be important. The compressive modulus of thermoplastic fibers is affected not only by their length and diameter, but also by the composition and properties of the polymer or polymers from which they are made, the shape and configuration of the fibers (e.g., concentric or eccentric, crimped or uncrimped), and like factors. Differences in the compressive modulus of these thermoplastic fibers can be used to alter the properties, and especially the density characteristics, of the respective absorbent members during preparation of the absorbent core.

As noted previously, in some absorbent cores according to the present invention, the fluid acquisition/distribution component can include hydrogel-forming absorbent to provide some fluid storage capacity for the core. In those instances, the fluid acquisition/distribution component can comprise up to about 50% hydrogel-forming absorbent polymer. Preferably, the fluid distribution component comprises up to about 30% hydrogel-forming absorbent polymer. Most preferably, the fluid distribution component comprises up to about 15% hydrogel-forming absorbent polymer.

The fluid acquisition/distribution component can also or alternatively comprise a polymeric foam material. Particularly suitable absorbent foams have been made from HIPEs. Though they differ with respect to certain properties from those foams discussed as being useful as the fluid storage component, the foams are open-celled, polymeric materials. See, for example, U.S. Pat. No. 5,260,345 (DesMarais et al), issued Nov. 9, 1993 and U.S. Pat. No. 5,268,224 (DesMarais et al), issued Dec. 7, 1993. These absorbent HIPE foams provide desirable fluid handling properties, including: (a) relatively good wicking and fluid distribution characteristics to transport the imbibed urine or other body fluid into the unused portion of the absorbent article to allow for subsequent gushes of fluid to be accommodated; and (b) a relatively high storage capacity with a relatively high fluid capacity under load, i.e. under compressive forces. These HIPE absorbent foams are also sufficiently flexible and soft so as to provide a high degree of comfort to the wearer of he absorbent article. See also U.S. Pat. No. 5,147,345 (Young et al), issued Sep. 15, 1992 and U.S. Pat. No. 5,318,554 (Young et al), issued Jun. 7, 1994, which discloses absorbent cores having a fluid acquisition/distribution component that can be a hydrophilic, flexible, open-celled foam such as a melamine-formaldehyde foam (e.g., BASOTECT made by BASF), and a fluid storage/redistribution component that is a HIPE-based absorbent foam.

These foam-based acquisition/distribution components should allow rapid fluid acquisition, as well as efficient partitioning or distribution of fluid to other components of the absorbent core having higher absorption pressures than the desorption pressure of the acquisition/distribution foam. This property of fluid desorption to other core components is important in enhancing the ability to accept repeated discharges or loadings of fluid and to maintain the skin dryness of the wearer. It also allows the acquisition/distribution foam to serve as a void volume reservoir, or buffer zone, to temporarily hold fluid that can be expressed from the storage components of the core when extraordinarily high pressures are encountered during use of the absorbent article.

In giving this fluid to other core components, these foam-based acquisition/distribution components should do so without densifying or collapsing. Foam-based acquisition/distribution components should also readily accept fluid, with or without the aid of gravity. Foam-based acquisition/distribution components should further provide good aesthetics, be soft and resilient in structure, and have good physical integrity in both wet and dry states.

Other foams useful as such acquisition/distribution components are described and claimed in co-pending U.S. Ser. No. 08/370,695, filed Jan. 10, 1995, by Stone, et al., which is incorporated by reference herein. These foams offer improved fluid retention and desorption (i.e., ability to relinquish fluid to other absorption components) properties, resulting from the processing conditions described therein. Briefly, the ability to provide these improved foams lies with the use of low shear conditions and a robust emulsifier system during HIPE processing.

Still other foams useful as the acquisition/distribution componenets are described and claimed in co-pending U.S. Ser. No. 08/520,793, filed Aug. 30, 1995 by DesMarais, which is incorporated herein by reference. These foams offer still better acquisition and desorption properties, again because of advancements made in the processing (e.g., low shear) and the emulsifier employed.

2. Upper Fluid Storage Component

The cores of the present invention optionally contain an upper fluid storage component that is relatively fluid permeable. The materials useful in the upper fluid storage component include materials that are capable of absorbing significant quantities of aqueous body fluids, with or without other optional components such as fibers, thermoplastic material, etc. The materials described as being useful as the swellable (also referred to herein as the "lower") fluid storage component(s) are also useful in the upper fluid storage component, though they need not swell in the z-direction as do the materials used as the lower fluid storage component.

Moreover, it is critical that the upper fluid storage component be sufficiently permeable to allow fluid to readily flow into the fluid acquisition zone. As such, preferred embodiments are those that use strips of storage material (e.g., fluid stable aggregates of hydrogel-forming polymer, discussed in detail below) and fiber/hydrogel-forming polymer blends such as the hydrogel-containing acquisition/distribution components discussed above.

If a non-continuous, patterned material is used as the upper fluid storage component, then the pattern should be chosen in a way that (1) sufficient void area between the storage regions is provided to allow fast passing of liquid into the fluid acuisition zone, but such that (2) enough area is covered by storage material to ensure good dryness. In this case it will generally be preferred to use about 50% of the area for storage.

If a continuous upper storage component is used, it is preferred that the hydrogel-forming polymer used in this component has a saline flow conductivity of at least about 50×10⁻⁷ cm³ sec/g, more preferably at least about 100×10⁻⁷ cm³ sec/g.

3. Swellable Fluid Storage Component

The materials useful as the swellable fluid storage component include materials that are capable of absorbing large quantities of aqueous body fluids, with or without other optional components such as fibers, thermoplastic material, etc. In addition to these properties, materials used in the fluid storage component must be capable of swelling in the z-direction upon imbibing fluid, so as to form the fluid acquisition zone. Materials capable of performing as the swellable fluid storage component(s) include substantially water-insoluble, water swellable absorbent polymer materials commonly referred to as "hydrogels", "hydrocolloids", or "superabsorbent" materials (for the purposes of the present invention, these materials are collectively referred to as "hydrogel-forming absorbent polymers"); and open-celled foam materials that remain in a collapsed (i.e., unexpanded) state until contacted with aqueous body fluids.

A principle function of these swellable fluid storage components is to absorb the discharged body fluids either directly or from other absorbent core components (e.g., the fluid acquisition/distribution component), and then retain such fluids, even when subjected to pressures normally encountered as a result of the wearer's movements. Another important funtion is the ability of the storage component(s) to swell to form the fluid acquisition zone. (It should be understood that the fluid storage component(s) can serve functions other than fluid storage and formation of a fluid acquisition zone, such as improving body fit.)

Regardless of the material used, it is preferred that the swellable storage component be capable of expanding in the z-direction from the dry, compressed state by at least 100% when fully saturated. Such z-directional expansion will effectively increase the volume of the fluid acquisition zone. However, those skilled in the art will recognize that the width and length of the fluid acquisition zone is also important to overall volume, and materials that do not swell in the z-direction by 100% may still be useful herein.

a. Hydrogel-forming Absorbent Polymers

When hydrogel polymers are used, an important aspect of these swellable fluid storage components according to the present invention is that they contain a relatively high concentration of the absorbent polymers. In order to provide relatively thin absorbent articles capable of absorbing and retaining large quantities of body fluids, it is desirable to increase the level of these hydrogel-forming absorbent polymers and to reduce the level of other components, in particular fibrous components. In measuring the concentration of hydrogel-forming absorbent polymer, the percent by weight of the hydrogel-forming polymer relative to the combined weight of hydrogel-forming polymer and any other components (e.g., fibers, thermoplastic material, etc.) that are present in the fluid storage component is used. With this in mind, the concentration of the hydrogel-forming absorbent polymers in a given fluid storage component according to the present invention can be in the range of from about 50 to 100%, preferably from about 60 to 100%, more preferably from about 70 to 100%, and most preferably from about 80 to 100%, by weight of the storage component.

A wide variety of hydrogel-forming absorbent polymers can be used in the fluid storage components of the present invention. These hydrogel-forming absorbent polymers have a multiplicity of anionic, functional groups, such as sulfonic acid, and more typically carboxy, groups. Examples of absorbent polymers suitable for use herein include those which are prepared from polymerizable, unsaturated, acid-containing monomers, including the olefinically unsaturated acids and anhydrides that contain at least one carbon to carbon olefinic double bond. More specifically, these monomers can be selected from olefinically unsaturated carboxylic acids and acid anhydrides, olefinically unsaturated sulfonic acids, and mixtures thereof. See U.S. Pat. No. 5,324,561 (Rezai et al), issued Jun. 23, 1994 (herein incorporated by reference), which describes suitable absorbent polymers and their preparation.

Preferred hydrogel-forming absorbent polymers for use in the present invention contain carboxy groups. These include hydrolyzed starch-acrylonitrile graft copolymers, partially neutralized starch-acrylonitrile graft copolymers, starch-acrylic acid graft copolymers, partially neutralized starch-acrylic acid graft copolymers, saponified vinyl acetate-acrylic ester copolymers, hydrolyzed acrylonitrile or acrylamide copolymers, slightly network crosslinked polymers of any of the foregoing copolymers, partially neutralized polyacrylic acid, and slightly network crosslinked polymers of partially neutralized polyacrylic acid. These polymers can be used either solely or in the form of a mixture of two or more different polymers. Examples of these polymer materials are disclosed in U.S. Pat. No. 3,661,875, U.S. Pat. No. 4,076,663, U.S. Pat. No. 4,093,776, U.S. Pat. No. 4,666,983, and U.S. Pat. No. 4,734,478.

Most preferred hydrogel-forming absorbent polymers for use in the present invention are slightly network crosslinked polymers of partially neutralized polyacrylic acids and starch derivatives thereof. Most preferably, the absorbent polymers comprise from about 50 to about 95%, preferably about 75%, neutralized, slightly network crosslinked, polyacrylic acid (i.e. poly (sodium acrylate/acrylic acid)). Processes for network crosslinking the polymers and typical network crosslinking agents are described in greater detail in the hereinbefore-referenced U.S. Pat. No. 4,076,663.

The hydrogel-forming absorbent polymers can be formed in any conventional manner. Preferred methods for forming these absorbent polymers are those that involve aqueous solution or other solution polymerization methods. See, for example, U.S. Reissue Pat. No. 32,649 (Brandt et al), reissued Apr. 19, 1988. While it is preferred that the absorbent polymers be manufactured using an aqueous solution polymerization process, it is also possible to carry out the polymerization process using multi-phase polymerization processing techniques such as inverse emulsion polymerization or inverse suspension polymerization procedures. See U.S. Pat. No. 4,340,706 (Obaysashi et al), issued Jul. 20, 1982, U.S. Pat. No. 4,506,052 (Flesher et al), issued Mar. 19, 1985, and U.S. Pat. No. 4,735,987 (Morita et al), issued Apr. 5, 1988, all of which are incorporated by reference, for processes involving inverse suspension polymerization. These absorbent polymers can synthesized or made in a variety of shapes and sizes, including fibers, granules, flakes or pulverulents. However, these absorbent polymers are most commonly supplied as absorbent particles or particulates.

One preferred class of hydrogel-forming absorbent polymers useful in the present invention are those which exhibit a high absorptive capacity. Absorptive capacity refers to the capacity of a given polymer material to absorb liquids with which it comes into contact. Absorptive capacity can vary significantly with the nature of the liquid being absorbed and with the manner in which the liquid contacts the polymer material. For purposes of this invention, Absorptive Capacity is defined in terms of the amount of Synthetic Urine absorbed by any given polymer material in terms of grams of Synthetic Urine per gram of polymer material in a procedure defined in the Test Methods section of U.S. Pat. No. 5,324,561 (Rezai et al), issued Jun. 23, 1994, which is incorporated by reference. Preferred absorbent polymers having a high absorptive capacity are those which have an Absorptive Capacity of at least about 20 grams, more preferably at least about 25 grams, of Synthetic Urine per gram of polymer material. Typically, these highly absorptive polymers have an Absorptive Capacity of from about 20 grams to about 70 grams of Synthetic Urine per gram of polymer material. Absorbent polymers having this relatively high absorptive capacity characteristic are especially useful in fluid storage components of the present invention since they hold desirably high amounts of discharged body exudates such as urine.

Another preferred class of hydrogel-forming absorbent polymers useful in the present invention are those having relatively high Saline Flow Conductivity (SFC) values and relatively high Performance Under Pressure (PUP) capacity. See copending U.S. application Ser. No. 219,574 (Goldman et al), filed Mar. 29, 1994, which is incorporated by reference, where SFC values and PUP capacity are defined and methods for measuring these parameters are provided. The Test Method section below also describes the test for determining SFC values. Absorbent polymers useful in the present invention can have SFC values of at least about 30×10⁻⁷ cm³ sec/g, preferably at least about 50×10⁻⁷ cm³ sec/g, and most preferably at least about 100×10⁻⁷ cm³ sec/g. Typically, these SFC values are in the range of from about 30 to about 1000×10⁻⁷ cm³ sec/g, more typically from about 50 to about 500×10⁻⁷ cm³ sec/g, and most typically from about 100 to about 350×10⁻⁷ cm³ sec/g. Absorbent polymers useful in the present invention generally have a PUP capacity at least about 23 g/g, preferably at least about 25 g/g, and most preferably at least about 29 g/g. Typically, these PUP capacity values are in the range of from about 23 to about 35 g/g, more typically from about 25 to about 33 g/g, and most typically from about 29 to about 33 g/g.

Surface crosslinking of the initially formed polymers is a preferred process for obtaining hydrogel-forming absorbent polymers having relatively high SFC and PUP capacity values. A number of processes for introducing surface crosslinks are disclosed in the art. These include those where: (i) a di- or poly-functional reagent(s) (e.g., glycerol, 1,3-dioxolan-2-one, polyvalent metal ions, polyquaternary amines) capable of reacting with existing functional groups within the hydrogel-forming absorbent polymer is applied to the surface of the hydrogel-forming absorbent polymer; (ii) a di- or poly-functional reagent that is capable of reacting with other added reagents and possibly existing functional groups within the hydrogel-forming absorbent polymer such as to increase the level of crosslinking at the surface is applied to the surface (e.g., the addition of monomer plus crosslinker and the initiation of a second polymerization reaction); (iii) no additional polyfunctional reagents are added, but additional reaction(s) is induced amongst existing components within the hydrogel-forming absorbent polymer either during or after the primary polymerization process such as to generate a higher level of crosslinking at or near the surface (e.g., heating to induce the formation of anhydride and or esters crosslinks between existing polymer carboxylic acid and/or hydroxyl groups and suspension polymerization processes wherein the crosslinker is inherently present at higher levels near the surface); and (iv) other materials are added to the surface such as to induce a higher level of crosslinking or otherwise reduce the surface deformability of the resultant hydrogel. Combinations of these surface crosslinking processes either concurrently or in sequence can also be employed. In addition to crosslinking reagents, other components can be added to the surface to aid/control the distribution of crosslinking (e.g., the spreading and penetration of the surface crosslinking reagents.) See copending U.S. application Ser. No. 219,574 (Goldman et al), filed Mar. 29, 1994, which is incorporated by reference,

Suitable general methods for carrying out surface crosslinking of hydrogel-forming absorbent polymers according to the present invention are disclosed in U.S. Pat. No. 4,541,871 (Obayashi), issued Sep. 17, 1985; published PCT application WO92/16565 (Stanley), published Oct. 1, 1992, published PCT application WO90/08789 (Tai), published Aug. 9, 1990; published PCT application WO93/05080 (Stanley), published Mar. 18, 1993; U.S. Pat. No. 4,824,901 (Alexander), issued Apr. 25, 1989; U.S. Pat. No. 4,789,861 (Johnson), issued Jan. 17, 1989; U.S. Pat. No. 4,587,308 (Makita), issued May 6, 1986; U.S. Pat. No. 4,734,478 (Tsubakimoto), issued Mar. 29, 1988; U.S. Pat. No. 5,164,459 (Kimura et. al.), issued Nov. 17, 1992; published German patent application 4,020,780 (Dahmen), published Aug. 29, 1991; and published European patent application 509,708 (Gartner), published Oct. 21, 1992; all of which are incorporated by reference. See also copending U.S. application Ser. No. 219,574 (Goldman et al), filed Mar. 29, 1994, which is incorporated by reference, and especially Examples 1 to 4.

While these hydrogel-forming absorbent polymers are preferably formed from the same monomers and have the same properties, this need not be the case. For example, some absorbent polymers can comprise a starch-acrylic acid graft copolymer while other absorbent polymers can comprise a slightly network crosslinked polymer of partially neutralized polyacrylic acid. Further, the absorbent polymers can vary in size, shape, absorptive capacity, or any other property or characteristic. In preferred embodiments of the present invention, the absorbent polymers consist essentially of slightly network crosslinked polymers of partially neutralized polyacrylic acid, each absorbent particle having similar properties.

One preferred fluid storage component according to the present invention having a relatively high concentration of these hydrogel-forming absorbent polymers is in the form of porous, absorbent macrostructures. These macrostructures are formed from a multiplicity of hydrogel-forming absorbent polymer particles. These macrostructures are capable of absorbing large quantities of aqueous body fluids (e.g., urine or menses) and then retaining such liquids under moderate pressures. Because they are formed from particles, these macrostructures have pores between adjacent particles. These pores are interconnected by intercommunicating channels such that the macrostructure is fluid permeable (i.e., has capillary transport channels).

Due to the bonds formed between the particles, the resultant aggregate macrostructures have improved structural integrity, increased fluid acquisition and distribution rates, and minimal gel-blocking characteristics. When wetted with aqueous fluids, the macrostructure swells generally isotropically even under moderate confining pressures, absorbs such fluids into the pores between the particles, and then imbibes such fluids into the particles. The isotropic swelling of the macrostructure allows the particles and the pores to maintain their relative geometry and spatial relationships even when swollen. Thus, the macrostructures are relatively "fluid stable" in that the particles do not dissociate from each other, thereby minimizing the incidence of gel blocking and allowing the capillary channels to be maintained and enlarged when swollen so that the macrostructure can acquire and transport subsequent loadings of liquid, even excess liquid. See U.S. Pat. No. 5,102,597 (Roe et al), issued Apr. 7, 1992, and U.S. Pat. No. 5,324,561 (Rezai et al), issued Jun. 23, 1994, which are incorporated by reference. As referred to herein, these macrostructures of interconnected particles are referred to as "fluid stable macrostuctures" or "fluid stable aggregates".

While these macrostructures can have a number of shapes and sizes, they are typically in the form of sheets, films, cylinders, blocks, spheres, fibers, filaments, or other shaped elements. These macrostructures will generally have a thickness or diameter between about 0.2 mm and about 10.0 mm. Preferably these macrostructures are in the form of sheets or strips. The terms "sheet" or "strips" as used herein describes macrostructures having a thickness of at least about 0.2 mm. The sheets or strips will preferably have a thickness between about 0.5 mm and about 10 mm, typically from about 1 mm to about 3 mm.

These macrostructures are formed by the joining or adhering together of adjacent particles. The adhesive agent is essentially the polymeric material that is present in the surface of these particles. When these particles are treated with a crosslinking agent and physically associated, the polymer material present in the surface of these particles is sufficiently plastic and cohesive (e.g., sticky) such that adjacent particles are adhered together, typically as discrete linking portions between the particles. The crosslinking reaction between the particles then sets this adhered structure.

In preparing these macrostructures, a crosslinking agent is used to provide crosslinking at the surface of the absorbent precursor particles. This typically occurs as a result of the crosslinking agent by reacting with the polymer material in these particles. Typically, the polymer material of the absorbent precursor particles has anionic, and preferably carboxy, functional groups that form a covalent, ester-type bond with the crosslinking agent. These portions of the absorbent particle that have been effectively crosslinked will swell less in the presence of aqueous (body) fluids relative to the other uncrosslinked portions of the particle.

Suitable crosslinking agents for this purpose can be nonionic and possess at least two functional groups per molecule capable of reacting with the carboxy group. See, for example, U.S. Pat. No. 5,102,597 (Roe et al), issued Apr. 7, 1992, which is incorporated by reference, and which discloses a variety of non ionic crosslinking agents. The particularly preferred nonionic crosslinking agent is glycerol. A preferred crosslinking agent for use in these macrostructures is an adduct of epichlorohydrin with certain types of monomeric or polymeric amines. See U.S. Pat. No. 5,324,561 (Rezai et al), issued Jun. 23, 1994, which is incorporated by reference and which discloses suitable cationic amino-epichlorohydrin adduct crosslinking agents. These amino-epichlorohydrin adducts, and especially the polymeric resin versions of these adducts, are preferred crosslinking agents because they react only with the polymer material at the surface precursor particles. In addition, the cationic functional groups (e.g., azetedinium groups) of these adducts, particularly polymeric resin versions, are believed to react very rapidly with the anionic, typically carboxy, functional groups of the polymer material of the absorbent particles, even at room temperature (e.g., at from about 18° to about 25° C.). Most preferred are certain polyamide-polyamine-epichlorohydrin resins particularly commercially marketed by Hercules Inc. under the trade name Kymene®. Especially useful are Kymene® 557H, Kymene® 557LX and Kymene® 557 Plus, which are the epichlorohydrin adducts of polyamide-polyamines that are the reaction products of diethylenetriamine and adipic acid. They are typically marketed in the form of aqueous solutions of the cationic resin material containing from about 10% to about 33% by weight of the resin active.

In preparing these porous, absorbent macrostructures, the absorbent particles are treated with the crosslinking agent, along with any other components or agents. For example, water is typically used with the crosslinking agent to form an aqueous treatment solution thereof. Water promotes the uniform dispersion of the crosslinking agent on the surface of the absorbent particles and causes permeation of the crosslinking agent into the surface regions of these particles. Water also promotes a stronger physical association between the treated precursor particles, providing greater integrity of the resultant interparticle bonded crosslinked aggregates. See U.S. Pat. No. 5,102,597 (Roe et al), issued Apr. 7, 1992 (nonionic crosslinking agents such as glycerol), and U.S. Pat. No. 5,324,561 (Rezai et al), issued Jun. 23, 1994 (cationic amino-epichlorohydrin adduct crosslinking agents), which are incorporated by reference.

It is particularly preferred that the treatment solution include a plasticizer, especially when cationic amino-epichlorohydrin adducts are used as the crosslinking agent. The preferred plasticizer is a mixture of glycerol and water, particularly when included as part of an aqueous treatment solution of the cationic amino-epichlorohydrin adduct, in a weight ratio of glycerol to water of from about 0.5:1 to about 2:1, preferably from about 0.8:1 to about 1.7:1. See U.S. Pat. No. 5,324,561 (Rezai et al), issued Jun. 23, 1994 which is incorporated by reference. Before, during, or after treatment with crosslinking agent, and optional plasticizer, the particles are physically associated together to form the aggregate macrostructures. A preferred method and apparatus for continuously forming these aggregate macrostructures into sheets is described in U.S. Pat. No. 5,324,561 (Rezai et al), issued Jun. 23, 1994 (cationic amino-epichlorohydrin adduct crosslinking agents), which are incorporated by reference. See especially FIG. 9 from this patent and its associated description.

Once the particles have been physically associated together to form an aggregate macrostructure, the crosslinking agent is reacted with the polymer material of the precursor particles, while maintaining the physical association of the particles, to provide effective surface crosslinking in the particles in the aggregate macrostructure. See U.S. Pat. No. 5,102,597 (Roe et al), issued Apr. 7, 1992 (nonionic crosslinking agents such as glycerol), and U.S. Pat. No. 5,324,561 (Rezai et al), issued Jun. 23, 1994 (cationic aminoepichlorohydrin adduct crosslinking agents), which are incorporated by reference. When amino-epichlorohydrin adducts are used as the crosslinking agent, this crosslinking reaction can occur at relatively low temperatures, including ambient room temperatures. Such ambient temperature curing is particularly desirable when the absorbent particles are treated a plasticizer, such as a mixture of water and glycerol. Curing at significantly above ambient temperatures can cause the plasticizer to be driven off due to its volatility, thus necessitating an additional step to plasticize the resulting aggregate macrostructure.

If desired, these macrostructures can include various types of fibers to act as reinforcing members. These include cellulose fibers, modified cellulose fibers, rayon, polypropylene, and polyester fibers such as polyethylene terephthalate (DACRON), hydrophilic nylon (HYDROFIL), and the like. Examples of other fiber materials are hydrophilized hydrophobic fibers, such as surfactant-treated or silica-treated thermoplastic fibers derived, for example, from polyolefins such as polyethylene or polypropylene, polyacrylics, polyamides, polystyrenes, polyurethanes and the like. In fact, hydrophilized hydrophobic fibers that are in and of themselves not very absorbent and which, therefore, do not provide webs of sufficient absorbent capacity to be useful in conventional absorbent structures, are suitable for use in these macrostructures by virtue of their good wicking properties. Synthetic fibers are generally preferred for use herein as the fiber component of the macrostructure. Most preferred are polyolefin fibers, preferably polyethylene fibers.

Other suitable swellable fluid storage components according to the present invention can be in the form of a layer of hydrogel-forming absorbent polymer particles contained between two other fibrous layers, e.g., a laminated fluid storage component. Suitable laminated fluid storage components according to the present invention can be prepared using procedures similar to those described in U.S. Pat. No. 4,260,443 (Lindsay et al); U.S. Pat. No. 4,467,012 (Pedersen et al), issued Aug. 21, 1984; U.S. Pat. No. 4,715,918 (Lang), issued Dec. 29, 1987; U.S. Pat. No. 4,851,069 (Packard et al), issued Jul. 25, 1989; U.S. Pat. No. 4,950,264 (Osborn), issued Aug. 21, 1990; U.S. Pat. No. 4,994,037 (Bernardin), issued Feb. 19, 1991; U.S. Pat. No. 5,009,650 (Bernardin), issued Apr. 23, 1991; U.S. Pat. No. 5,009,653 (Osborn), issued Apr. 23, 1991; U.S. Pat. No. 5,128,082 (Makoui), Jul. 7, 1992; U.S. Pat. No. 5,149,335 (Kellenberger et al), issued Sep. 22, 1992; and U.S. Pat. No. 5,176,668 (Bernardin), issued Jan. 5, 1993 (all of which are incorporated by reference). These laminated fluid storage components can be in the form of thermally bonded fibrous layers, adhesively bonded fibrous layers (e.g., glue bonding between the fibrous layers or between the fibrous layers and the hydrogel-forming absorbent polymer particles), or fibrous layers that are held together by hydrogen bonding (e.g., by spraying the fibrous layers with water followed by compaction).

If desired, the above macrostructures or absorbent particles can be attached to a substrate to form the fluid storage components. The substrate can provide a variety of functions, including: (1) improving the distribution of fluids to be absorbed by the macrostructure/particles; and (2) supporting the macrostructure/particles by providing additional integrity, especially in the situation where the absorbent particles begin to swell after absorbing fluid. The substrate can be made from various materials known in the art such as cellulose fibers, nonwoven webs, tissue webs, foams, polyacrylate fibers, apertured polymeric webs, synthetic fibers, metallic foils, elastomers, and the like. Most such substrate materials can distribute fluids to, as well as support, the macrostructure/particles. Preferably, the substrate is comprised of cellulosic material or a material having cellulosic functionality. Preferred substrates for distributing fluids are cellulosic materials, fibrous webs, cellulosic fibrous webs, tissues, solid foams, cellulosic foams, and polyvinyl alcohol foams. Preferred substrates for supporting the macrostructure/particles are tissues, cellulosic materials, fibrous webs, nonwoven webs, fabrics, cellulosic fibrous webs, solid foams, cellulosic foams, and polyvinyl alcohol foams.

The substrate is preferably flexible and pliable to encourage such properties in the resulting absorbent composite with the macrostructure/particles. The substrate can be substantially resilient and non-stretchable, or it can be stretchable or deformable to a varying extent in response to forces exerted normal to and in the plane of the surface of the substrate. The thickness and basis weight (weight per unit area of substrate) of the substrate material can vary depending on the type of substrate and properties desired. The substrate can comprise a plurality of individual sheets, or plies, of a particular substrate material, or a combination of one or more substrate layers in a laminate. One such suitable substrate is a Bounty® sheet having a thickness of from about 0.02 mm to about 1.2 mm, more preferably from about 0.3 mm to about 0.8 mm; and a basis weight of from about 5 g/m² to about 100 g/m², more preferably from about 10 g/m² to about 60 g/m², and most preferably from about 15 g/m² to about 40 g/m². Another suitable substrate is a cellulose foam having a dry compressed thickness of from about 0.5 mm to about 3.0 mm, more preferably from about 0.8 mm to about 2.0 mm; a wet expanded thickness of from about 0.8 mm to about 6.0 mm, more preferably from about 1.0 mm to about 5.0 mm; and a basis weight of from about 50 g/m² to about 2,000 g/m², more preferably from about 100 g/m² to about 1,000 g/m².

Substrates suitable for supporting the macrostructure/particles typically have a dry tensile strength of from about 500 g/in to about 8,000 g/in, more preferably from about 1,000 g/in to about 3,000 g/in; a wet tensile strength of from about 200 g/in to about 5,000 g/in, more preferably from about 400 g/in to about 1,000 g/in; and a wet burst strength of from about 100 g to about 2,000 g, more preferably from about 200 g to about 1,000 g. Preferred substrates of this type include cellulosic fibrous webs such as paper towels and tissues such those disclosed in U.S. Pat. No. 3,953,638, issued Apr. 27, 1976, U.S. Pat. No. 4,469,735, issued Sep. 4, 1984, U.S. Pat. No. 4,468,428, issued Aug. 28, 1984, and U.S. Pat. No. 4,986,882, issued Jan. 22, 1991, all of which are incorporated by reference.

The porous absorbent macrostructure/particles can be attached to the substrate by a variety of chemical, physical, and adhesive agents. Adhesive agents for attaching the macrostructure/particles to substrate include glues and hot melt adhesives. Preferably, the bonding between the substrate and macrostructure/particles is achieved by depositing the precursor absorbent particles on the substrate, treating the deposited particles with the solution comprising a crosslinking agent and then curing the treated particles/substrate as previously. In a preferred embodiment of this method, a cellulosic substrate (e.g., paper towel) is used. The precursor absorbent particles are then deposited on this cellulosic substrate. A treatment solution comprising an amino-epichlorohydrin adduct, preferably a polymeric epichlorohydrin-polyamide/polyamine wet strength resin such Kymene®, is then applied (e.g., sprayed) on the cellulosic substrate and the absorbent particles. The treated substrate/particles are then cured at ambient temperatures such that the particles are bonded to the cellulosic substrate.

To enhance the overall flexibility of the cores of the present invention, the fluid stable macrostructures may be slitted so as to be discontinuous. That is, the macrostructures may each be cut at various locations to form slits through the entire thickness (i.e., z-direction) of the structure. Such macrostructures are described in copending U.S. application Ser. Nos. 08/142,258 (filed Oct. 22, 1993 by Hsueh et al.), 08/550,181 (filed Oct. 30, 1995 by Rezai et al.), and 08/550,185 (filed Oct. 30, 1995 by Dierckes et al.), all of which are incorporated by reference herein. Upon stretching the slitted macrostructures in the y-direction, a "netted" material results. The open spaces between the hydrogel-containing continuous portion permits freer swelling of the hydrogel structure, and also increases permeability through this component. Such materials are especially useful as the material for the upper fluid storage zone.

The porous, absorbent macrostructures useful as swellable fluid storage components according to the present invention can also be enclosed or enveloped within a tissue. Such tissue envelopes can keep loose absorbent particles from migrating within the absorbent core and can provide additional structural integrity to the macrostructure.

Regardless of the nature of the absorbent material utilized, it is important that the swellable storage material be restrained from swelling to a significant degree into the fluid acquisition zone (i.e., in the x- and/or y-directions, particularly toward the interior of the absorbent core), while being free to swell in the z-direction. This will result in a larger acquisition zone to accept fluid gushes. As is discussed with regard to FIGS. 5, swelling into the fluid acquistion zone can be prevented by, for example, use of adhesive spot bonds. This may be particularly beneficial when discrete particles of absorbent polymer are used as the swellable fluid storage material. In some instances, the arrangement of the various core materials will provide the desired restrained swelling.

b. Foam Materials

As indicated above, foam materials useful as the swellable, absorbent storage component of the present invention should, in addition to offering adequate fluid storage capacity, be capable of existing in a collapsed, or thin, state until contacted with a body fluid. The ability to remain in such as state is important in providing thin diapers that appeal to the consumer. The use of these foam materials provides the added advantage of swelling almost entirely in the z-direction. That is, upon imbibing fluid, the foams swell significantly in the z-direction, while essentially maintaining their length and width dimensions. This is important in that it allows for efficient formation of the fluid acquisition zone.

Representative foam materials that are useful in the present invention are those described in U.S. Pat. No. 5,387,207 ("'207 patent"), issued Feb. 7, 1995 to Dyer et al., which is incorporated herein by reference. Briefly, that patent describes polymeric foams derived from emulsions that have a relatively small amount of an oil phase (including the polymerizable monomers) and a relatively large amount of an aqueous phase. (Such emulsions are commonly referred to as high internal phase emulsions, or HIPEs.) These HIPE-derived foams are rendered hydrophilic by agents remaining after polymerization, or by post-polymerization treatment with a surfactant. The foams described in the '207 patent are open-celled. That is, the individual cells (also referred to as pores) that represent the space occupied by individual water droplets in the emulsion are interconnected by numerous small holes. These small holes in the walls of the cells allow fluid to transfer from one cell to another, throughout the entire foam structure.

The ability of the foams to remain in the "thin-until-wet" state is believed to be due to the capillary forces within the foam, particularly the foam's capillary pressure. To remain in the collapsed state until wetted, the capillary pressures within the foam must be equivalent or greater than the forces exerted by the elastic recovery or modulus of the foam polymer, which work to "spring" the foam back to its uncompressed thickness. Parameters that affect capillary pressure include capillary suction specific surface area, foam density, fluid surface tension and average cell size. Parameters that affect the modulus of the foams include monomer content comprising the polymer, as well as residual oil-soluble emulsifiers which tend to plasticize the polymer, thereby reducing polymer modulus. A complete list of preferred ranges for these parameters, as well as a discussion of other important properties of the foams, is set forth in the patent.

Co-pending U.S. application Ser. No. 08/563,866, which is incorporated by reference herein, was filed Nov. 29, 1995 by DesMarais et al., and also describes expandable foams that are useful in the present invention. Though these foams are also prepared from HIPEs, the preparation of emulsions with higher water-oil ratios provides even higher porosity, lower density structures. These foams are particularly preferred as the fluid storage material when use of foams are desired.

Other foam materials useful as the swellable storage material include compressed cellulosic foams. Such foams are described in European Patent Publication 293,208, published by Lion Corp. Cellulose foams useful herein are commercially available from several companies, including Spontex, Toray and 3M. These cellulose foam materials when supplied as compressed sponges expand rapidly upon wetting, thus creating the fluid acquisition zone.

D. Fluid Acquisition Zone

The fluid acquisition zone, formed in-part by the swollen storage component(s), creates a void space beneath the core materials, particularly the lower (if more than one is present) acquisition/distribution layer. Because of the void space created by the acquisition zone, the absorbent cores according to the present invention can more easily handle "gushes" of discharged body fluids. This is especially important as portions of the absorbent core become saturated from prior multiple discharges of such fluids.

As can be seen by refering to the drawings, the fluid acquisition zone has three dimensions. The width (y-direction) and length (x-direction) of the acquisition zone is generally defined as the void area created by the swellable fluid storage component(s). Where two lateral storage components are spaced apart, the width of the acquisition zone is the gap between these components. The length in this case will be determined by the length of the swellable storage components. The depth of the zone will be the height (z-direction) of the swollen storage components.

The fluid acquisition zone may be of irregular shape in the x-y directions, though generally rectangular is preferred. Further, while the volume of the acquisition zone required will vary according to various factors (e.g., the rate of absorbency of the storage material; the absorbency rate and capacity of the acquistion material; the size of the wearer; etc.), it is preferred that the acquisition zone have a volume, when the storage material is wetted, of at least about 30 cc, preferrably at least about 50 cc, and more preferably at least about 75 cc. Of course, when in the dry state, the acquisition zone volume will be significantly smaller. Those skilled in the art will recognize that measuring acquisition zone volumes will be imprecise, given the nature of the materials employed as the storage and acquisition/distribution components. As such, the preferred volume ranges listed are illustrative only, and are not intended to limit the scope of the invention.

E. Topsheets

Topsheets useful in absorbent articles of the present invention are compliant, soft feeling, and non-irritating to the wearer's skin. These topsheets are fluid pervious to permit body fluids to readily penetrate through its thickness. A suitable topsheet can be manufactured from a wide range of materials such as woven and nonwoven materials; polymeric materials such as apertured formed thermoplastic films, apertured plastic films, and hydroformed thermoplastic films; porous foams; reticulated foams; reticulated thermoplastic films; and thermoplastic scrims. Suitable woven and nonwoven materials can be comprised of natural fibers (e.g., wood or cotton fibers), synthetic fibers (e.g., polymeric fibers such as polyester, polypropylene, or polyethylene fibers) or from a combination of natural and synthetic fibers.

Preferred topsheets for use in the present invention are selected from high loft nonwoven topsheets and aperture formed film topsheets. Apertured formed films are especially preferred for the topsheet because they are pervious to body fluids and yet nonabsorbent and have a reduced tendency to allow fluids to pass back through and rewet the wearer's skin. Thus, the surface of the formed film that is in contact with the body remains dry, thereby reducing body soiling and creating a more comfortable feel for the wearer. Suitable formed films are described in U.S. Pat. No. 3,929,135 (Thompson), issued Dec. 30, 1975; U.S. Pat. No. 4,324,246 (Mullane, et al), issued Apr. 13, 1982; U.S. Pat. No. 4,342,314 (Radel. et al), issued Aug. 3, 1982; U.S. Pat. No. 4,463,045 (Ahr et al), issued Jul. 31, 1984; and U.S. Pat. No. 5,006,394 (Baird), issued Apr. 9, 1991. Each of these patents are incorporated herein by reference. Particularly preferred microapertured formed film topsheets are disclosed in U.S. Pat. No. 4,609,518 (Curro et al), issued Sep. 2, 1986 and U.S. Pat. No. 4,629,643 (Curro et al), issued Dec. 16, 1986, which are incorporated by reference. The preferred topsheet for use in catamenial products of the present invention is the formed film described in one or more of the above patents and marketed on sanitary napkins by The Procter & Gamble Company of Cincinnati, Ohio as "DRI-WEAVE®."

The body surface of the formed film topsheet can be hydrophilic so as to help body fluids to transfer through the topsheet faster than if the body surface was not hydrophilic so as to diminish the likelihood that fluid will flow off the topsheet rather than flowing into and being absorbed by the absorbent structure. In a preferred embodiment, surfactant is incorporated into the polymeric materials of the formed film topsheet such as is described in U.S. patent application Ser. No. 07/794,745, "Absorbent Article Having A Nonwoven and Apertured Film Coversheet" filed on Nov. 19, 1991 by Aziz, et al., which is incorporated by reference. Alternatively, the body surface of the topsheet can be made hydrophilic by treating it with a surfactant such as is described in the above referenced U.S. Pat. No. 4,950,254, incorporated herein by reference.

F. Backsheets

Backsheets useful in absorbent articles of the present invention are typically impervious to body fluids and are preferably manufactured from a thin plastic film, although other flexible fluid impervious materials may also be used. As used herein, the term "flexible" refers to materials that are compliant and will readily conform to the general shape and contours of the human body. The backsheet prevents body fluids absorbed and contained in the absorbent core from wetting articles that contact the such as pants, pajamas, undergarments, and the like. The backsheet can comprise a woven or nonwoven material, polymeric films such as thermoplastic films of polyethylene or polypropylene, or composite materials such as a film-coated nonwoven material. Preferably, the backsheet is a polyethylene film having a thickness of from about 0.012 mm (0.5 mil) to about 0.051 mm (2.0 mils). Exemplary polyethylene films are manufactured by Clopay Corporation of Cincinnati, Ohio, under the designation P18-0401 and by Ethyl Corporation, Visqueen Division, of Terre Haute, Indiana, under the designation XP-39385. The backsheet is preferably embossed and/or matte finished to provide a more clothlike appearance. Further, the backsheet can permit vapors to escape from the absorbent core (i.e., breathable) while still preventing body fluids from passing through the backsheet.

Particularly desirable backsheets can be made from a structural elastic-like film (SELF) web. A structural elastic-like film web is an extensible material that exhibits an elastic-like behavior in the direction of elongation without the use of added elastic materials. The SELF web includes a strainable network having at least two contiguous, distinct, and dissimilar regions. One of the regions is configured so that it will exhibit resistive forces in response to an applied axial elongation in a direction parallel to the predetermined axis before a substantial portion of the other region develops significant resistive forces to the applied elongation. At least one of the regions has a surface-path length that is greater than that of the other region as measured substantially parallel to the predetermined axis while the material is in an untensioned condition. The region exhibiting the longer surface-path length includes one or more deformations that extend beyond the plane of the other region. The SELF web exhibits at least two significantly different stages of controlled resistive force to elongation along at least one predetermined axis when subjected to an applied elongation in a direction parallel to the predetermined axis. The SELF web exhibits first resistive forces to the applied elongation until the elongation of the web is sufficient to cause a substantial portion of the region having the longer surface-path length to enter the plane of applied elongation, whereupon the SELF web exhibits second resistive forces to further elongation. The total resistive forces to elongation are higher than the first resistive forces to elongation provided by the first region. SELF webs suitable for the present invention are more completely described in the copending, commonly assigned U.S. patent application Ser. No. 08/203,456 entitled "Absorbent Article with Multiple Zone Structural Elastic-Like Film Web Extensible Waist Feature" filed by Donald C. Roe, et al. on Feb. 24, 1994, which is incorporated herein by reference.

G. Absorbent Articles

The absorbent articles of the present invention generally comprise: (1) a topsheet; (2) a backsheet; and (3) an absorbent core positioned between the topsheet and the backsheet. As used herein, the term "absorbent article" refers to articles that absorb and contain body fluids, and more specifically refers to articles that are placed against or in proximity to the body of the wearer to absorb and contain the various fluids discharged from the body. Additionally, "disposable" absorbent articles are those which are intended to be discarded after a single use (i.e., the original absorbent article in its whole is not intended to be laundered or otherwise restored or reused as an absorbent article, although certain materials or all of the absorbent article may be recycled, reused, or composted). A preferred embodiment of a disposable absorbent article according to the present invention is a diaper. As used herein, the term "diaper" refers to a garment generally worn by infants and incontinent persons that is worn about the lower torso of the wearer. It should be understood, however, that the present invention is also applicable to other absorbent articles such as incontinent briefs, incontinent pads, training pants, diaper inserts, catamenial pads, sanitary napkins, facial tissues, paper towels, and the like.

The absorbent core used in the absorbent articles of the present invention comprises at least one swellable fluid storage component, as previously described, that is located relatively remote from the wearer. This is so the fluid that is temporarily located in the fluid acquistion zone is remote from the wearer, so as to avoid rewet and to enhance acquistions rates.

In a preferred embodiment, the absorbent core comprises two swellable fluid storage components that are laterally spaced apart. By "laterally spaced apart" is meant that there is a gap between the these fluid storage components. When these laterally spaced storage components swell in the z-direction upon absorbing body fluid, this gap between the fluid storage components, together with the acquisition/distribution layer positioned above, defines the fluid acquisition zone for receiving discharged body fluids. At least a portion of this fluid acquisition zone comprises a void space underneath the overlying acquisition/distribution component. Because of the void space in this acquisition zone, the absorbent core according to the present invention can more easily handle "gushes" of discharged body fluids. This is especially important as portions of the absorbent core become saturated from prior multiple discharges of such fluids.

Absorbent cores according to the present invention further comprise a fluid acquisition/distribution component that is capable of transporting the discharged body fluids to other components in the absorbent core. This fluid acquisition/distribution component is at least partially positioned underneath and typically proximate to the fluid discharge region of the core so as to be able to receive these discharged body fluids. At least a portion of this fluid acquisition/distribution component is also positioned above the upper storage material (when included) or the swellable storage material, which allows the transfer of fluids from the fluid distribution component to the fluid acquisition zone, so the fluid acquisition/distribution component can receive additional discharges of body fluid.

Where the acquisition/distribution material is fibrous, it is preferred that the basis weight be in the range of from about 0.08 g/sq.in. to about 0.30 g/sq.in., more preferably from about 0.08 g/sq.in to about 0.15 g/sq.in.; and that the density be in the range of from about 0.05 g/cc to about 0.30 g/cc; more preferably from about 0.05 g/cc to about 0.15 g/cc.

Certain absorbent core designs according to the present invention may comprise two strips of storage material in the upper fluid storage zone, positioned between an upper and lower fluid acquisition/distribution components. A portion of this storage material is also positioned in the fluid discharge region of the core. This upper fluid storage zone is in fluid communication with the fluid acquisition/distribution components so as to be able to store a portion of the acquired body fluids. The swellable materials useful as the fluid storage components that form the fluid acquisition zone may also be useful in the upper fluid storage component. Nonetheless, it is not necessary that this component be capable of swelling in the z-direction upon imbibing fluid. Thus, the skilled artisan will recognize that any material capable of absorbing a significant amount of fluid can be utilized as this upper storage component.

An embodiment of the an absorbent article in the form of a diaper 10 having one such absorbent core according to the present invention is shown in FIG. 1. FIG. 1 is a top plan view of diaper 10 in a flat-out, uncontracted state (i.e., with any elastic-induced contraction removed) having a topsheet 12, a backsheet 14, and an absorbent core indicated generally as 18 that is positioned between topsheet 12 and backsheet 14. Topsheet 12 is shown as being transparent so as to better illustrate the various components of absorbent core 18.

As also shown in FIG. 1, diaper 10 has a front waistband region 22, a back waistband region 24, a crotch region 26 and a periphery 28 that is defined by the outer edge of backsheet 14 and which has longitudinal edges designated 30 and end edges designated as 32. The longitudinal axis of diaper 10 essentially runs parallel to longitudinal edges 30, while the transverse axis essentially runs parallel to end edges 32. The waistband regions 22 and 24 comprise those upper portions of the diaper 10, which when worn, encircle the waist of the wearer. The crotch region 26 is that portion of the diaper 10 between waistband regions 22 and 24, and comprises that portion of the diaper 10 which when worn, is positioned between the legs of the wearer and covers the lower torso of the wearer. Thus, the crotch region 26 defines the area of typical liquid deposition for a diaper 10 or other disposable absorbent article.

Topsheet 12 and backsheet 14 can be associated together in any suitable manner. As used herein, the term "associated" encompasses configurations where topsheet 12 is directly joined to backsheet 14 by affixing the topsheet directly to the backsheet, and configurations where the topsheet is indirectly joined to the backsheet by affixing the topsheet to intermediate members which in turn are affixed to the backsheet. Preferably, the topsheet 12 and backsheet 14 are affixed directly to each other by attachment means (not shown) such as an adhesive or any other attachment means as known in the art. For example, a uniform continuous layer of adhesive, a patterned layer of adhesive, or an array of separate lines or spots of adhesive may be used to affix topsheet 12 to backsheet 14. As shown in FIG. 1, topsheet 12 has a smaller size configuration than backsheet 14. However, topsheet 12 and backsheet 14 can both have the same, or a similar, size configuration (i.e., are coextensive) such they are joined together at periphery 28 of diaper 10. The size of the backsheet 14 is dictated by the size of the absorbent core 18 and the exact diaper design selected. In the embodiment shown in FIG. 1, the backsheet 14 has an hourglass-shaped configuration. However, other configuration such as rectangular, I-shaped and the like are also suitable.

Although not shown, diaper 10 can have elastic members that exert a contracting force on the diaper so that it configures more closely and more comfortably to the wearer. These elastic members can be assembled in a variety of well known configurations, those described generally in U.S. Pat. No. 3,860,003 (Buell), issued Jan. 14, 1975, which patent is incorporated by reference. The elastic members can be disposed adjacent the periphery 28 of the diaper 10, preferably along each longitudinal edge 30, so that the elastic members tend to draw and hold the diaper 10 against the legs of the wearer. Alternatively, the elastic members can be disposed adjacent either or both of the lateral edges 32 of diaper 10 to provide a waistband as well as or rather than leg cuffs. See, for example, U.S. Pat. No. 4,515,595 (Kievit et al), issued May 7, 1985, which is incorporated by reference. The elastic members are secured to the diaper 10 in an elastically contractible condition so that in a normally unrestrained configuration, these elastic members effectively contract or gather the diaper 10. The elastic members can be secured in an elastically contractible condition in at least two ways. For example, the elastic members can be stretched and secured while the diaper 10 is in an uncontracted condition. Alternatively, the diaper 10 can be contracted, for example, by pleating, and the elastic members secured and connected to the diaper 10 while they are in their unrelaxed or unstretched condition. The elastic members can extend essentially the entire length of the diaper 10 in the crotch region 26, or alternatively can extend the entire length of the diaper 10, or any other length suitable to provide an elastically contractible line. The length of these elastic members is typically dictated by the diaper's design.

Referring to FIG. 1 and especially FIG. 2, absorbent article 10 has a fluid acquisition/distribution component 42 located adjacent topsheet 12. The core 18 further comprises two fluid storage components 34 and 36 in the form of rectangular strips that comprise hydrogel-forming absorbent polymer and are positioned adjacent backsheet 14. These fluid storage components 34 and 36 (which are preferably wrapped with tissues 35 and 37, respectively, as depicted, when they are formed from absorbent hydrogel forming polymer) are laterally spaced apart and define the fluid acquisition zone identified generally as 38. This fluid acquisition zone 38 is generally in the fluid discharge region of diaper 10.

The continuous fluid acquisition/distribution component 42, as depicted in FIG. 2, is in the form of of chemically stiffened fibers (preferably containing little or no hydrogel-forming polymer). This fluid acquisition/distribution component 42 has lateral portion 45 and 47.

Upon first exposure to aqueous body fluids, storage components 34 and 36 begin to swell, increasing in caliper by at least 2 mm when fully saturated. This increase in caliper increases the void volume of the acquisition zone 38. Consequently, the absorbent article is better able to handle subsequent "gushes" of aqueous body fluids. Preferably, the storage components will expand by at least 100% in the z-direction. Of course, because the length and width of the fluid acquisition zone will also affect the core's void volume, 100% z-direction expansion may not be required in such cores.

FIG. 3 depicts an article simlar to that depicted in FIG. 2, but further comprises an upper acquisition component positioned between the topsheet and the acquisition/distribution component described in FIG. 2 as 42. Refering to FIG. 3, absorbent core 118 has an upper fluid acquisition/distribution component 141 located adjacent topsheet 112. The core further comprises two fluid storage components 134 and 136 in the form of rectangular strips that comprise hydrogel-forming absorbent polymer and are positioned adjacent backsheet 114. These fluid storage components are each respectively wrapped in a fluid pervious paper tissue 135 and 137, as shown specifically in FIG. 3. These wrapped fluid storage components 134 and 136 are laterally spaced apart and define the fluid acquisition zone identified generally as 138. This fluid acquisition zone 138 is generally in the fluid discharge region of diaper 110.

The upper continuous fluid acquisition component 141, as depicted in FIG. 3, is in the form of chemically stiffened fibers (preferably containing little or no hydrogel-forming polymer). This upper fluid acquisition component 141 has lateral portions 145 and 147.

Continuous lower fluid acquisition/distribution component 142 is between the upper fluid aquisition component 141 and the fluid storage components 134 and 136, and is in the form of chemically stiffened fibers or regular fluff pulp fibers containing about 20% to 50% of hydrogel-forming polymer having a saline flow conductivity of at least about 50×10⁻⁷ cm³ sec/g, preferably at least about 100×10⁻⁷ cm³ sec/g, that is wrapped in a paper tissue 143. The middle portion 144 of this lower fluid acquisition/distribution component 142 is positioned above the fluid acquisition zone 138. This lower fluid acquisition/distribution component 142 also has lateral portion 146 and 148. Lateral portion 146 is positioned above and in fluid communication with fluid storage component 136, while lateral portion 148 is positioned above and in fluid communication with fluid storage component 134. In preferred embodiments, lower acquisition/distribution component 142 will cover the lower fluid storage components, as well as the fluid acquisition zone formed by the storage components.

Upon first exposure to aqueous body fluids, storage components 134 and 136 begin to swell, increasing in caliper by at least 2 mm when fully saturated. This increase in caliper increases the void volume of the acquisition zone 138. Consequently, the absorbent article is better able to handle subsequent "gushes" of aqueous body fluids. Preferably, the storage components will expand by at least 100% in the z-direction. Of course, because the length and width of the fluid acquisition zone will also affect the core's void volume, 100% z-direction expansion may be required in such cores.

FIG. 4 depicts yet another embodiment, wherein absorbent core 218 has an upper fluid acquisition/distribution component 241 located adjacent topsheet 212. The core further comprises an upper fluid storage component 260 positioned beneath upper acquisition component 241 and above lower acquisition/distribution component 242. Upper fluid storage component 260 is in the form of a netted fluid stable aggregate of hydrogel-forming polymer, and is wrapped with tissue 261.

The core further comprises two fluid storage components 234 and 236 in the form of rectangular strips that comprise hydrogel-forming absorbent polymer or a thin-until-wet polymeric foam and are positioned adjacent backsheet 214. These fluid storage components are each respectively wrapped in a fluid pervious paper tissue 235 and 237, as shown specifically in FIG. 4. These wrapped fluid storage components 234 and 236 are laterally spaced apart and define the fluid acquisition zone identified generally as 238. This fluid acquisition zone 238 is generally in the fluid discharge region of diaper 210.

The upper continuous fluid acquisition component 241, as depicted in FIG. 4, is in the form of chemically stiffened fibers. This upper fluid acquisition component 241 has lateral portion 245 and 247.

Continuous lower fluid acquisition/distribution component 242 is in the form a fibrous web that is preferably made out of chemically stiffened cellulose. Where the web contains hydrogel-forming polymer, it may optionally be wrapped in tissue (the tissues are not depicted in FIG. 4). The middle portion 244 of this fluid acquisition/distribution component 242 is positioned above the fluid acquisition zone 238. This fluid acquisition/distribution component 242 also has lateral portions 246 and 248. Lateral portion 246 is positioned above and in fluid communication with fluid storage component 236, while lateral portion 248 is positioned above and in fluid communication with fluid storage component 234.

Upon first exposure to aqueous body fluids, storage components 234 and 236 begin to swell, increasing in caliper by at least 2 mm when fully saturated. This increase in caliper increases the void volume of the acquisition zone 238. Consequently, the absorbent article is better able to handle subsequent "gushes" of aqueous body fluids. Preferably, the storage components will expand by at least 100% in the z-direction. Of course, because the length and width of the fluid acquisition zone will also affect the core's void volume, 100% z-direction expansion may be required in such cores.

FIG. 5 shows a cross section of an absorbent article 310 having a topsheet 312, a backsheet 314 and an alternative absorbent core 318 positioned between the topsheet and the backsheet. In this embodiment, an upper fluid storage component is in the form four separate strips, designated as 362, 363, 364, and 365, of fluid stable aggregates of hydrogel-forming polymer, which are spaced apart and run longitudinally in the article. Strips 362, 363, 364, and 365 are wrapped by tissue 372, 373, 374, and 375, respectively, and are located under upper fluid acquisition component 341, which is formed from chemically stiffened fibers.

This alternative absorbent core 318 also has to two fluid storage components 334 and 336 that comprise hydrogel-forming absorbent polymer or a thin-until-wet polymeric foam, and are adjacent backsheet 314. These fluid storage components 334 and 336 are laterally spaced apart and define, in part, the fluid acquisition zone identified generally as 338. A paper tissue layer 343 is positioned between these fluid storage components 334 and 336 and the lower fluid acquisition/distribution component 342, and is preferably a Bounty® sheet having a thickness of from about 0.02 mm to about 1.2 mm, more preferably from about 0.3 mm to about 0.8 mm; and a basis weight of from about 5 g/m² to about 100 g/m², more preferably from about 10 g/m² to about 60 g/m², and most preferably from about 15 g/m² to about 40 g/m². This fluid acquisition/distribution component 342 has a middle portion 344 positioned above the fluid acquisition zone 338 and has lateral portion 346 and 348. Lateral portion 346 is positioned above and in fluid communication with fluid storage component 336, while lateral portion 348 is positioned above and in fluid communication with fluid storage component 334.

Tissue 343 is adhesively bonded to backsheet 314 at the points indicated by 366 and 368.

Upon first exposure to aqueous body fluids, storage components 334 and 336 begin to swell, increasing in caliper by at least 2 mm when fully saturated. Adhesive bonds 366 and 368 prevent lateral expansion of storage components 334 and 336 into acquisition zone 338. The increase in caliper increases the void volume of the acquisition zone 338. Consequently, the absorbent article is better able to handle subsequent "gushes" of aqueous body fluids.

FIG. 6 shows a cross section of an absorbent article 410 having a topsheet 412, a backsheet 414 and an alternative absorbent core 418 positioned between the topsheet and the backsheet. This alternative absorbent core 418 also has two fluid storage components 434 and 436 that comprise hydrogel-forming absorbent polymer or a polymeric absorbent foam, and are above and adjacent backsheet 414. These fluid storage components 434 and 436 are laterally spaced apart and define the fluid acquisition zone identified generally as 438. Where comprised of hydrogel-forming absorbent polymer, these fluid storage components 434 and 436 are further bonded to substrates 435 and 437, respectively. Regardless of the material comprising fluid storage components 434 and 436, both are folded in a c-shape as shown in FIG. 6.

The upper fluid storage component is again depicted as comprising four spaced-apart fluid stable aggregates of hydrogel-forming absorbent polymer, designated as strips 462, 463, 464, and 465, positioned between upper fluid acquisition component 441 (which preferably comprises chemically stiffened fibers) and lower fluid acquisition/distribution component 442.

Fluid distribution component 442 is positioned above and in fluid communication with fluid storage components 434 and 436 and above fluid acquisition zone 438. A paper tissue layer 443 is positioned over fluid storage components 434 and 236 and under lower fluid distribution component 442.

Upon first exposure to aqueous body fluids, storage components 434 and 436 begin to swell, increasing in caliper by at least 2 mm when fully saturated. Substrate layers 435 and 437 prevent lateral expansion of storage components 434 and 436 into acquisition zone 438. The increase in caliper increases the void volume of the acquisition zone 438. Consequently, the absorbent article is better able to handle subsequent "gushes" of aqueous body fluids.

H. Test Method--Saline Flow Conductivity (SFC)

This test determines the Saline Flow Conductivity (SFC) of the gel layer formed from hydrogel-forming absorbent polymer that is swollen in Jayco synthetic urine under a confining pressure. The objective of this test is to assess the ability of the hydrogel layer formed from a hydrogel-forming absorbent polymer to acquire and distribute body fluids when the polymer is present at high concentrations in an absorbent member and exposed to usage mechanical pressures. Darcy's law and steady-state flow methods are used for determining saline flow conductivity. (See, for example, "Absorbency," ed. by P. K. Chatterjee, Elsevier, 1985, Pages 42-43 and "Chemical Engineering Vol. II", Third Edition, J. M. Coulson and J. F. Richardson, Pergamon Press, 1978, Pages 125-127.) The hydrogel layer used for SFC measurements is formed by swelling a hydrogel-forming absorbent polymer in Jayco synthetic urine for a time period of 60 minutes. The hydrogel layer is formed and its flow conductivity measured under a mechanical confining pressure of 0.3 psi (about 2 kPa). Flow conductivity is measured using a 0.118M NaCl solution. For a hydrogel-forming absorbent polymer whose uptake of Jayco synthetic urine versus time has substantially leveled off, this concentration of NaCl has been found to maintain the thickness of the hydrogel layer substantially constant during the measurement. For some hydrogel-forming absorbent polymers, small changes in hydrogel-layer thickness can occur as a result of polymer swelling, polymer deswelling, and/or changes in hydrogel-layer porosity. A constant hydrostatic pressure of 4920 dyne/cm² (5 cm of 0.118M NaCl) is used for the measurement.

Flow rate is determined by measuring the quantity of solution flowing through the hydrogel layer as a function of time. Flow rate can vary over the duration of the measurement. Reasons for flow-rate variation include changes in the thickness of the hydrogel layer and changes in the viscosity of interstitial fluid, as the fluid initially present in interstitial voids (which, for example, can contain dissolved extractable polymer) is replaced with NaCl solution. If flow rate is time dependent, then the initial flow rate, typically obtained by extrapolating the measured flow rates to zero time, is used to calculate flow conductivity. The saline flow conductivity is calculated from the initial flow rate, dimensions of the hydrogel layer, and hydrostatic pressure. For systems where the flow rate is substantially constant, a hydrogel-layer permeability coefficient can be calculated from the saline flow conductivity and the viscosity of the NaCl solution.

A suitable apparatus 610 for this test is shown in FIG. 7. This apparatus includes a constant hydrostatic head reservoir indicated generally as 612 that sits on a laboratory jack indicated generally as 614. Reservoir 612 has lid 616 with a stoppered vent indicated by 618 so that additional fluid can be added to reservoir 612. An open-ended tube 620 is inserted through lid 616 to allow air to enter reservoir 612 for the purpose of delivering fluid at a constant hydrostatic pressure. The bottom end of tube 620 is positioned so as to maintain fluid in cylinder 634 at a height of 5.0 cm above the bottom of hydrogel layer 668 (see FIG. 8).

Reservoir 612 is provided with a generally L-shaped delivery tube 622 having an inlet 622a that is below the surface of the fluid in the reservoir. The delivery of fluid by tube 622 is controlled by stopcock 626. Tube 622 delivers fluid from reservoir 612 to a piston/cylinder assembly generally indicated as 628. Beneath assembly 628 is a support screen (not shown) and a collection reservoir 630 that sits on a laboratory balance 632.

Referring to FIG. 7, assembly 628 basically consists of a cylinder 634, a piston generally indicated as 636 and a cover 637 provided with holes for piston 636 and delivery tube 622. As shown in FIG. 7, the outlet 622b of tube 622 is positioned below the bottom end of tube 620 and thus will also be below the surface of the fluid (not shown) in cylinder 634. As shown in FIG. 8, piston 636 consists of a generally cylindrical LEXAN® shaft 638 having a concentric cylindrical hole 640 bored down the longitudinal axis of the shaft. Both ends of shaft 638 are machined to provide ends 642 and 646. A weight indicated as 648 rests on end 642 and has a cylindrical hole 648a bored through the center thereof.

Inserted on the other end 646 is a generally circular Teflon piston head 650 having an annular recess 652 in the bottom thereof. Piston head 650 is sized so as to slidably move inside cylinder 634. As particularly shown in FIG. 9, piston head 650 is provided with four concentric rings of twenty-four cylindrical holes each indicated generally as 654, 656, 658, and 660. As can be seen in FIG. 9, concentric rings 654 to 660 fit within the area defined by recess 652. The holes in each of these concentric rings are bored from the top to bottom of piston head 650. The holes in each ring are spaced by approximately 15 degrees and offset by approximately 7.5 degrees from the holes in adjacent rings. The holes in each ring have a progressively smaller diameter going inwardly from ring 654 (0.204 inch diameter) to ring 660 (0.111 inch diameter). Piston head 650 also has cylindrical hole 662 bored in the center thereof to receive end 646 of shaft 638.

As shown in FIG. 8, a fritted circular glass disc 664 fits within recess 652. Attached to bottom end of cylinder 634 is a No. 400 mesh stainless steel cloth screen 666 that is biaxially stretched to tautness prior to attachment. The sample of hydrogel-forming absorbent polymer indicated as 668 is supported on screen 666.

Cylinder 634 is bored from a transparent LEXAN® rod or equivalent and has an inner diameter of 6.00 cm (area=28.27 cm²), a wall thickness of approximately 0.5 cm, and a height of approximately 6.0 cm. Piston head 650 is machined from a solid Teflon rod. It has a height of 0.625 inches and a diameter that is slightly less than the inner diameter of cylinder 634, so that it fits within the cylinder with minimum wall clearances, but still slides freely. Recess 652 is approximately 56 mm in diameter by 4 mm deep. Hole 662 in the center of the piston head 650 has a threaded 0.625 inch opening (18 threads/inch) for end 646 of shaft 638. Fritted disc 664 is chosen for high permeability (e.g., Chemglass Cat No. CG-201-40, 60 mm diameter; X-Coarse Porosity) and is ground so that it fits snugly within recess 652 of piston head 650, with the bottom of the disc being flush with the bottom of the piston head. Shaft 638 is machined from a LEXAN® rod and has an outer diameter of 0.875 inches and an inner diameter of 0.250 inches. End 646 is approximately 0.5 inches long and is threaded to match hole 662 in piston head 650. End 642 is approximately an inch long and 0.623 inches in diameter, forming an annular shoulder to support the stainless steel weight 648. Fluid passing through the hole 640 in shaft 638 can directly access the fritted disc 664. The annular stainless steel weight 648 has an inner diameter of 0.625 inches, so that it slips onto end 642 of shaft 638 and rests on the annular shoulder formed therein. The combined weight of fritted glass disc 664, piston 636 and weight 648 equals 596 g, which corresponds to a pressure of 0.3 psi for an area of 28.27 cm². Cover 637 is machined from LEXAN® or its equivalent and is dimensioned to cover the top of cylinder 634. It has an 0.877 inch opening in the center thereof for shaft 638 of piston 636 and a second opening near the edge thereof for delivery tube 622.

The cylinder 634 rests on a 16 mesh rigid stainless steel support screen (not shown) or equivalent. This support screen is sufficiently permeable so as to not impede fluid flow into the collection reservoir 630. The support screen is generally used to support cylinder 634 when the flow rate of saline solution through assembly 628 is greater than about 0.02 g/sec. For flow rates less than about 0.02 g/sec, it is preferable that there be a continuous fluid path between cylinder 634 and the collection reservoir. This can be accomplished by replacing the support screen, collection reservoir 630, and analytical balance 632 with analytical balance 716, reservoir 712, fritted funnel 718, and the respective connecting tubes and valves of apparatus 710 (see FIG. 10), and positioning cylinder 634 on the fritted disc in fritted funnel 718.

Jayco synthetic urine used in this method is prepared by dissolving a mixture of 2.0 g KCL, 2.0 g Na₂ SO₄, 0.85 g NH₄ H₂ PO₄, 0.15 g (NH₄)₂ HPO₄, 0.19 g CaCl₂, and 0.23 g MgCl₂ to 1.0 liters with distilled water. The salt mixture can be purchased from Endovations, Reading, Pa. (cat No. JA-00131-000-01).

The 0.118M NaCl solution is prepared by dissolving 6.896 g NaCl (Baker Analyzed Reagent or equivalent) to 1.0 liters with distilled water.

An analytical balance 632 accurate to 0.01 g (e.g., Mettler PM4000 or equivalent) is typically used to measure the quantity of fluid flowing through the hydrogel layer 668 when the flow rate is about 0.02 g/sec or greater. A more accurate balance (e.g., Mettler AE200 or equivalent) can be needed for less permeable hydrogel layers having lower flow rates. The balance is preferably interfaced to a computer for monitoring fluid quantity versus time.

The thickness of hydrogel layer 668 in cylinder 634 is measured to an accuracy of about 0.1 mm. Any method having the requisite accuracy can be used, as long as the weights are not removed and the hydrogel layer is not additionally compressed or disturbed during the measurement. Using a caliper gauge (e.g., Manostat 15-100-500 or equivalent) to measure the vertical distance between the bottom of the stainless steel weight 648 and the top of cover 637, relative to this distance with no hydrogel layer 668 in cylinder 634 is acceptable. Also acceptable is the use of a depth gauge (e.g., Ono Sokki EG-225 or equivalent) to measure the position of piston 636 or stainless steel weight 648 relative to any fixed surface, compared to its position with no hydrogel layer in cylinder 634.

The SFC measurement is performed at ambient temperature (i.e., 20°-25° C.) and is carried out as follows:

0.9 gm aliquot of hydrogel-forming absorbent polymer (corresponding to a basis weight of 0.032 gm/cm²) is added to cylinder 634 and distributed evenly on screen 666. For most hydrogel-forming absorbent polymers, moisture content is typically less than 5%. For these, the quantity of hydrogel-forming absorbent polymer to be added can be determined on a wet-weight (as is) basis. For hydrogel-forming absorbent polymers having a moisture content greater than about 5%, the added polymer weight should be corrected for moisture (i.e., the added polymer should be 0.9 g on a dry-weight basis). Care is taken to prevent hydrogel-forming absorbent polymer from adhering to the cylinder walls. Piston 636 (minus weight 648) with disc 664 positioned in recess 652 of piston head 650 is inserted into cylinder 634 and positioned on top of the dry hydrogel-forming absorbent polymer 668. If necessary, piston 636 can be turned gently to more-uniformly distribute the hydrogel-forming absorbent polymer on screen 666. Cylinder 634 is the covered with cover 637 and weight 648 is then positioned on end 642 of shaft 638.

A fritted disc (coarse or extra coarse) having a diameter greater than that of cylinder 634 is positioned in a wide/shallow flat-bottomed container that is filled to the top of the fritted disc with Jayco synthetic urine. The piston/cylinder assembly 628 is then positioned on top of this fritted glass disc. Fluid from the container passes through the fritted disc and is absorbed by the hydrogel-forming absorbent polymer 668. As the polymer absorbs fluid, a hydrogel layer is formed in cylinder 634. After a time period of 60 minutes, the thickness of the hydrogel layer is determined. Care is taken that the hydrogel layer does not lose fluid or take in air during this procedure.

The piston/cylinder assembly 628 is then transferred to apparatus 610. The support screen (not shown) and any gap between it and the piston/cylinder assembly 628 is presaturated with saline solution. If the fritted funnel 718 of the PUP apparatus 710 is used to support cylinder 634, the surface of the fritted funnel should be minimally elevated relative to the height of the fluid in the collection reservoir, with valves between the fritted funnel and the collection reservoir being in the open position. (The fritted funnel elevation should be sufficient such that fluid passing through the hydrogel layer does not accumulate in the funnel.)

The SFC measurement is initiated by adding NaCl solution through hole 640 in shaft 638 in order to expel air from piston head 650 and then turning stopcock 626 to an open position so that delivery tube 622 delivers fluid to cylinder 634 to a height of 5.0 cm above the bottom of hydrogel layer 668. Although the measurement is considered to have been initiated (t_(o)) at the time NaCl solution is first added, the time at which a stable hydrostatic pressure, corresponding to 5.0 cm of saline solution, and a stable flow rate is attained (t_(s)) is noted. (The time t_(s) should typically be about one minute or less.) The quantity of fluid passing through hydrogel layer 668 versus time is determined gravimetrically for a time period of 10 minutes. After the elapsed time, piston/cylinder assembly 628 is removed and the thickness of hydrogel layer 668 is measured. Generally the change in thickness of the hydrogel layer is less than about 10%.

In general, flow rate need not be constant. The time-dependent flow rate through the system, F_(s) (t) is determined, in units of g/sec, by dividing the incremental weight of fluid passing through the system (in grams) by incremental time (in seconds). Only data collected for times between t_(s) and 10 minutes is used for flow rate calculations. Flow rate results between t_(s) and 10 minutes is used to calculate a value for F_(s) (t=0), the initial flow rate through the hydrogel layer. F_(s) (t=0) is calculated by extrapolating the results of a least-squares fit of F_(s) (t) versus time to t=0.

For a layer having a very high permeability (e.g., a flow rate greater than ˜2 g/sec), it may not be practical to collect fluid for the full 10 minute time period. For flow rates greater than ˜2 g/sec, the time of collection can be shortened in proportion to the flow rate.

For some hydrogel-forming absorbent polymers having extremely low permeability, absorption of fluid by the hydrogel competes with transport of fluid through the hydrogel layer and either there is no flow of fluid through the hydrogel layer and into the reservoir or, possibly, there is a net absorption of fluid out of the PUP reservoir. For these extremely low permeability hydrogel layers, it is optional to extend the time for Jayco SynUrine absorption to longer periods (e.g., 16 hours).

In a separate measurement, the flow rate through apparatus 610 and the piston/cylinder assembly 628 (F_(a)) is measured as described above, except that no hydrogel layer is present. If F_(a) is much greater than the flow rate through the system when the hydrogel layer is present, F_(s), then no correction for the flow resistance of the SFC apparatus and the piston/cylinder assembly is necessary. In this limit, F_(g) =F_(s), where F_(g) is the contribution of the hydrogel layer to the flow rate of the system. However if this requirement is not satisfied, then the following correction is used to calculate the value of F_(g) from the values of F_(s) and F_(a) :

    F.sub.g =(F.sub.a ×F.sub.s)/(F.sub.a -F.sub.s)

The Saline Flow Conductivity (K) of the hydrogel layer is calculated using the following equation:

    K={F.sub.g (t=0)×L.sub.0 }/{ρ×A×ΔP},

where F_(g) (t=0) is the flow rate in g/sec determined from regression analysis of the flow rate results and any correction due to assembly/apparatus flow resistance, L₀ is the initial thickness of the hydrogel layer in cm, ρ is the density of the NaCl solution in gm/cm³. A is the area of the hydrogel layer in cm², ΔP is the hydrostatic pressure in dyne/cm², and the saline flow conductivity, K, is in units of cm³ sec/gm.

The average of three determinations should be reported.

For hydrogel layers where the flow rate is substantially constant, a permeability coefficient (κ) can be calculated from the saline flow conductivity using the following equation:

    κ=Kη,

where η is the viscosity of the NaCl solution in poise and the permeability coefficient, κ, is in units of cm².

The following is an example of how SFC is calculated according to the present invention:

The measured value of F_(a) is 412 g/min=6.87 g/sec. For a single determination on the particulate hydrogel-forming polymer sample 3-5 (Example 3), the extrapolated value for F_(s) (t=0) is 33.9 g/min=0.565 g/sec, with a very-low ratio of slope:intercept of 9×10⁻⁵ sec⁻¹. Correcting for apparatus resistance:

    F.sub.g =(6.87×0.565)÷(6.87-0.565)=0.616 g/sec

Given a 0.118M saline density of 1.003 g/cm³ (CRC Handbook of Chemistry and Physics, 61st Edition) a hydrogel-layer thickness of 1.134 cm, a hydrogel layer area of 28.27 cm², and a hydrostatic pressure of 4920 dyne/cm².

    K=(0.616×1.134)/(1.003×28.27×4920)=5.0×10.sup.-6 cm.sup.3 sec/gm

Considering the substantially constant flow rate and given a 0.118M saline viscosity of 0.01015 poise (CRC Handbook of Chemistry and Physics, 61st Edition):

    κ=Kη=(5.0×10.sup.-6)×0.01015=5.1×10.sup.-8 cm.sup.2 

What is claimed is:
 1. An absorbent core capable of absorbing discharged body fluids, the absorbent core comprising.(1) a fluid acquisition/distribution component capable of receiving aqueous fluids in the fluid discharge region of the absorbent core, wherein the fluid acquisition/distribution component either(a) contains less than about 50%, by weight of the acquisition/distribution component, of absorbent hydrogel-forming polymer; or (b) contains at least 50%, by weight of the acquisition/distribution component, of absorbent hydrogel-forming polymer, the absorbent hydrogel-forming polymer having a Saline Flow Conductivity (SFC) value of at least about 50×10⁻⁷ cm³ sec/g; and (2) at least one fluid storage component positioned at least partially underneath and in fluid communication with the fluid acquisition/distribution component, said at least one fluid storage component being capable of swelling in the z-direction when saturated with aqueous body fluids so as to form a fluid acquisition zone, said fluid acquisition zone comprising a space substantially devoid of absorbent material adjacent said at least one fluid storage component, and said at least one fluid storage component being restrained so as to prevent substantial swelling of said at least one fluid storage component toward said fluid acquisition zone and so that any swelling of said at least one fluid storage component toward said fluid acquisition zone will be substantially less than said swelling of aid at least one fluid storage component in said z-direction.
 2. The absorbent core of claim 1, wherein the fluid acquisition/distribution component contains less than about 30%, by weight of the acquisition/distribution component, of absorbent hydrogel-forming polymer.
 3. The absorbent core of claim 1, wherein the fluid acquisition/distribution component comprises absorbent hydrogel-forming polymer having a Saline Flow Conductivity value of at least about 100×10⁻⁷ cm³ sec/g.
 4. The absorbent core of claim 1, wherein the core further comprises an upper fluid acquisition component above the fluid acquisition/distribution component.
 5. The absorbent core of claim 4, wherein the upper fluid acquisition component comprises chemically stiffened cellulose fibers and the fluid acquisition/distribution component contains less than about 50%, by weight of the acquisition/distribution component, of absorbent hydrogel-forming polymer.
 6. The absorbent core of claim 5, wherein the upper fluid acquisition component contains essentially no absorbent hydrogel-forming polymer.
 7. The absorbent core of claim 1 wherein the fluid storage component comprises two strips that run longitudinally in the absorbent core, wherein the components are laterally spaced apart so as to form the fluid acquistion zone upon contact with aqueous body fluids.
 8. An absorbent core capable of absorbing discharged body fluids, the absorbent core having a fluid discharge region and comprising:(1) an upper fluid acquisition/distribution component capable of receiving aqueous fluids in the fluid discharge region of the absorbent core; (2) an upper fluid storage component at least partially underneath and in fluid communication with said upper fluid acquisition/distribution component, said upper fluid storage component being permeable to the flow of aqueous body fluids in the z-direction; (3) a lower fluid acquisition/distribution component capable of acquiring and transporting aqueous body fluids, the lower fluid acquisition/distribution component being positioned at least partially underneath and in fluid communication with the upper fluid storage component; and (4) at least one lower fluid storage component positioned at least partially underneath and in fluid communication with the lower fluid acquisition/distribution component, said at least one lower fluid storage component being capable of swelling in the z-direction when saturated with aqueous body fluids so as to form a fluid acquisition zone, said fluid acquisition zone comprising a space substantially devoid of absorbent material adjacent said at least one lower fluid storage component, and said at least one lower fluid storage component being restrained so as to prevent substantial swelling of said at least one lower fluid storage component toward said fluid acquisition zone and so that any swelling of said at least one lower fluid storage component toward said fluid acquisition zone will be substantially less than said swelling of said at least one lower fluid storage component in said z-direction.
 9. The absorbent core of claim 8 wherein the core comprises two lower fluid storage components.
 10. The absorbent core of claim 9 wherein the two lower fluid storage components are strips that run longitudinally in the absorbent core, wherein the components are laterally spaced apart so as to form the fluid acquistion zone upon contact with aqueous body fluids.
 11. The absorbent core of claim 10 wherein both laterally spaced apart lower fluid storage components comprise: a) a fluid stable macrostructure comprising interconnected, hydrogel-forming absorbent polymer particles; and b) a substrate to which the interconnected, hydrogel-forming absorbent polymer particles are bonded.
 12. The absorbent core of claim 11 wherein each of the laterally spaced apart lower fluid storage components is c-folded longitudinally to form a two layer component.
 13. The absorbent core of claim 8 wherein the lower fluid storage component(s) comprises from about 50 to 100%, by weight of the respective storage component, of hydrogel-forming absorbent polymer.
 14. The absorbent core of claim 13 wherein the lower fluid storage component(s) comprises from about 70 to 100%, by weight of the storage component, of hydrogel-forming absorbent polymer.
 15. The absorbent core of claim 13 wherein the hydrogel-forming absorbent polymer has an Absorptive Capacity of at least about 25 grams of Synthetic Urine per gram of polymer material.
 16. The absorbent core of claim 13 wherein the hydrogel-forming absorbent polymer has a Saline Flow Conductivity value of at least about 30×10⁻⁷ cm³ sec/g and a Performance Under Pressure capacity value of at least about 23 g/g.
 17. The absorbent core of claim 8, wherein the upper fluid storage component contains an absorbent hydrogel-forming polymer, the hydrogel forming polymer having a saline flow conductiviy of at least about 100×10⁻⁷ cm³ sec/g.
 18. The absorbent core of claim 8, wherein the upper fluid storage component(s) of the absorbent core comprises an open cell foam that will rapidly expand upon wetting.
 19. The absorbent core of claim 18 wherein the open cell foam material is a compressed cellulose sponge.
 20. The absorbent core of claim 9 wherein the lower fluid storage components comprise a collapsable polymeric foam material derived from a high internal phase water in oil emulsion.
 21. The absorbent core of claim 20 wherein the polymeric foam material has:A) a specific surface area per foam volume of at least about 0.025 m² /cc; B) a capillary suction specific surface area of at least about 3 m² /g; C) a foam density, as measured in the collapsed state, of from about 0.1 g/cc 5 to about 0.2 g/cc; D) an average cell size of less than about 50 μm; E) a ratio of expanded to collapsed thickness of at least about 6:1; and F) a free absorbent capacity of from about 55 to about 100 ml of synthetic urine, per gram of foam.
 22. The absorbent core of claim 8 wherein the upper fluid acquisition/distribution layer comprises chemically stiffened cellulosic fibers.
 23. The absorbent core of claim 22 wherein the upper fluid acquisition/distribution layer comprises chemically stiffened cellulosic fibers that are thermally bonded with a thermoplastic material.
 24. The absorbent core of claim 23 wherein the upper fluid acquisition/distribution layer comprises essentially no hydrogel-forming absorbent polymer.
 25. The absorbent core of claim 8 wherein the lower fluid acquisition/distribution layer comprises comprises up to about 30%, by weight of the fluid acquisition/distribution component, hydrogel-forming absorbent polymer.
 26. The absorbent core of claim 8 wherein the upper fluid storage component comprises from about 70 to 100%, by weight, of hydrogel-forming absorbent polymer.
 27. The absorbent core of claim 26 wherein the upper fluid storage component comprises a fluid stable macrostructure comprising interconnected, hydrogel-forming absorbent polymer particles.
 28. The absorbent core of claim 27 wherein the fluid stable macrostructure further comprises a substrate to which the interconnected, hydrogel-forming absorbent polymer particles are bonded, and wherein further the fluid stable macrostructure is slitted through its entire thickness so as to be discontinuous.
 29. The absorbent core of claim 8 wherein the upper fluid storage component comprises a plurality of laterally spaced apart strips that run longitudinally in the absorbent core, where each strip comprises a) a fluid stable macrostructure comprising interconnected, hydrogel-forming absorbent polymer particles; and b) a substrate to which the interconnected, hydrogel-forming absorbent polymer particles are bonded.
 30. The absorbent core of claim 29, wherein the upper fluid storage component comprises four laterally spaced apart strips.
 31. An absorbent core capable of absorbing discharged aqueous body fluids, said absorbent core having a fluid discharge region and comprising:(1) an upper fluid acquisition/distribution component capable of receiving aqueous fluids, the upper fluid acquisition/distribution component being positioned in the fluid discharge region of the absorbent core and comprising chemically stiffened cellulosic fibers that are thermally bonded with a thermoplastic material; (2) an upper fluid storage component at least partially underneath and in fluid communication with said upper fluid acquisition/distribution component, said upper fluid storage component being permeable to the flow of aqueous body fluids in the z-direction; (3) a lower fluid acquisition/distribution component capable of acquiring and transporting aqueous body fluids, the lower fluid acquisition/distribution component being positioned at least partially underneath and in fluid communication with the upper fluid storage component, wherein the lower fluid acquisition/distribution component comprises from 0 to about 30%, by weight, of a hydrogel-forming polymer polymer; and (4) two lower fluid storage components positioned at least partially underneath and in fluid communication with the lower fluid acquisition/distribution component, the two lower fluid storage components being capable of swelling in the z-direction when saturated with aqueous body fluids so as to form a fluid acquisition zone, said fluid acquisition zone comprising a space substantially devoid of absorbent material between said two lower fluid storage components, and both of said lower fluid storage components being restrained so as to prevent substantial swelling of said two lower fluid storage components toward said fluid acquisition zone and so that any swelling of said two lower fluid storage components toward said fluid acquisition zone will be substantially less than said swelling of said two lower fluid storage components in said z-direction, wherein the lower storage components are in the form of spaced apart strips that run longitudinally in the absorbent core, and wherein both lower fluid storage components comprise material selected from the group consisting of a fluid stable macrostructure of interconnected, hydrogel-forming absorbent polymer particles and a collapsable, open celled foam.
 32. The absorbent core of claim 31, wherein the upper fluid storage component comprises four spaced-apart strips that run longitunidally in the core.
 33. The absorbent core of claim 31, wherein the upper fluid storage component contains an absorbent hydrogel-forming polymer, the hydrogel forming polymer having a saline flow conductiviy of at least about 100×10⁻⁷ cm³ sec/g.
 34. An absorbent article for absorbing discharged aqueous body fluids, the absorbent article comprising:(A) a topsheet; (B) a backsheet; and (C) the absorbent core of claim 1 located between the topsheet and backsheet.
 35. An absorbent article for absorbing discharged aqueous body fluids, the absorbent article comprising:(A) a topsheet; (B) a backsheet; and (C) the absorbent core of claim 8 located between the topsheet and backsheet.
 36. An absorbent article for absorbing discharged aqueous body fluids, the absorbent article comprising:(A) a topsheet; (B) a backsheet; and (C) the absorbent core of claim 10 located between the topsheet and backsheet.
 37. An absorbent article for absorbing discharged aqueous body fluids, the absorbent article comprising:(A) a topsheet; (B) a backsheet; and (C) the absorbent core of claim 20 located between the topsheet and backsheet.
 38. An absorbent article for absorbing discharged aqueous body fluids, the absorbent article comprising:(A) a topsheet; (B) a backsheet; and (C) the absorbent core of claim 30 located between the topsheet and backsheet.
 39. An absorbent article for absorbing discharged aqueous body fluids, the absorbent article comprising:(A) a topsheet; (B) a backsheet; and (C) the absorbent core of claim 31 located between the topsheet and backsheet.
 40. An absorbent article for absorbing discharged aqueous body fluids, the absorbent article comprising:(A) a topsheet; (B) a backsheet; and (C) the absorbent core of claim 32 located between the topsheet and backsheet. 